Pokémon Movepool Oddities & Explanations

[CPU]

You would think that's it, but, by generation:
Charmander, Charmeleon, Charizard, Blastoise. Why do any of the Chars get it, and why Charmander but not Squirtle if we had to pick a pre-evo?
Typhlosion(and H), Feraligatr. Because screw Meganium, amirite?
Sceptile, Combusken, Blaziken, Swampert. Fighting-type justifies Combusken but no other pre-evos, sure. But Swampert breaks the trend of only bipeds getting the move, and seems...less suited than most of the other options(this is pure vibes). Sceptile is at least the first grass-type starter to get it.
Monferno, Infernape. Again, fighting makes sense. Why is Empoleon the first water-type to miss the move?
Pignite, Emboar. Again, fighting.
Chesnaught, Delphox. Psychic and fighting, fine. But the dark-type projectile-focused ninja doesn't get it?
Decidueye H(not A), Incineroar. Basically a fire/fighting, fine. Though again, both of the other starters are more focused on ranged attacks with mental powers than he is, which is odd.
Rillaboom, Cinderace. ???
And no starters in SV get it, which makes the first time the fully-evolved fire type cannot learn the move.

Like I said, I think there are various explanations for what does and doesn't get the move, I just don't think any of those explanations are consistent with each other.

Charmander and Charmeleon learning the move is new to Gen 9; a quick comparison of the move's distribution from Gen 8 to 9 shows some slightly increased availability (in addition to the littler Chars, both Gravelers and Elekid get the move for the first time in Gen 9, for example).

I think there's definitely some intention for the move to be a Ki blast; having hands (or at least arm-adjacent appendages that one can bring together) is nearly a prerequisite, though one that some Fighting types skirt around (the Swords of Justice and Falinks).

The distribution requirements, inconsistently applied though they may be, seem to me to be:

1. Hands/Arms to, well, focus the blast
2. Being fully evolved to have the mental fortitude to fire a Ki bomb
3. Being Fighting type (or Fighting-adjacent, which is my logic for Graveler/Elekid, among others) may let a Pokémon bypass the first two requirements (being a Legendary/Mythical might help too)

I don't see a Pokémon being focused around ranged/projectile attacks impacting what learns it except in some rare cases (Clawitzer, maybe Yveltal?). Addressing some of the starter anomalies you brought up:

• Swampert is capable of attacking bipedally, at least, and it learns Hammer Arm by level (I actually associate this move with Swampert for whatever reason).
• Not exactly sure why Empoleon doesn't get it; I guess GF decided its claws on its flippers weren't handsy enough.
• Greninja not learning Focus Blast does seem weird at first glance, but as a ninja it's more suited to covert attacks, not a big flashy energy shot. Its Dark type may also provide a reason why it doesn't get the move; perhaps it's not "pure of heart" enough (other Dark Pokémon can learn the move, though [again, Yveltal], so this is a flimsy conjecture).
• Immediate counterpoint: Dark-type Incineroar. But, as you mentioned, it's a Fighting type in spirit, and as it's based on a wrestling heel, it could also just be "playing the villain."
• I would assume that A-Decidueye, Inteleon, and Meowscarada don't get Focus Blast for the same logic that Greninja doesn't: they're designed around sneaky, out-of-sight attacks, despite having hands. Hard to say if H-Decidueye gets it because it's a more aggressive attacker or because it's a Fighting type.
• I have no idea on Quaquaval. Maybe it's too caught up in dancing to be able to charge a Ki blast

Just gonna close this out with my personal weirdest pick for what learns Focus Blast:

 

[pika pal]

(I wrote this post Friday but it failed to post due to an internet outage. Pardon any retreaded talk like Starter distribution)

Focus Blast has always felt like the most "for gameplay" move of the prominent lot to me, given how long it spent as a TM exclusive and remains a near-exclusive move even now, almost as if it was initially designed as a move that most mons can only learn artificially.

The additions are strange to me though. The monkeys might be a sort of DB reference (not even specifically to Kamehameha, DBZ and Super have multiple instances of Kiai's or focused glares with power enough behind them to launch an opponent). Yveltal I'd chalk up to legendary privilege and it draining energy just by existing near things, but I do still find it odd it was literally the only natural learner for so long (not even its counterpart Xerneas). And as for Clawitzer, it's bizarre because Aura Sphere is a superior move thanks to it always having Mega Launcher, and despite being blocked by Bulletproof, FB is not considered a Ball/Pulse move so it has neither a major gameplay benefit (the SpD proc is not worth the lower accuracy for identical BP) nor the loose theming behind it (i.e. it and Blastoise simply getting most of the non-signature Mega Launcher moves).

Meanwhile the mons that CAN'T learn it are relatively few but feel like they're excluded for reasons of Gameplay balance: Just looking over Gen 1, you have mons like Primeape, Slowbro, and Snorlax that aren't exactly depicted as bright or focused on much (unless being really angry at 1 thing or wanting only to sleep regardless of circumstance is focus), while Pidgeot is excluded from it despite the move being practically made for its Mega's kit by the time we hit Gen 6 and even the base form seeming fit to it with its sharp vision (Keen Eye and Dex entries about how it hunts). There are other oddities on top of that like the starters, with Venusaur never having the move, Blastoise being the sole recipient in its line, and Charizard initially being the sole user before Gen 9 specifically added the TM to Charmander and Charmeleon, but not to any other early stages.

Speaking of Starters: Meganium (not TOO weird since it's a less combative Mon) is left out for Johto while Typhlosion and Feraligatr get it pretty consistently via TM... except in ZA for the latter (and Feraligatr has it in SwSh as well as SV, so they had to consciously take that out regardless of when movepools and development happened). Gen 3 it's available to the whole trio, then Gens 3-7 are pretty reasonable with it only going to Fighting or Fighting-adjacent starters like Incineroar... and then we hit Galar where Cinderace and Rillaboom can learn it, but not Inteleon, the Sniper that has to be able to focus on a target so precisely they can shoot with centimeter precision at a great distance. This is another case where I question what isn't getting it more than what is, as this feels like a move you would only keep away from Inteleon for gameplay reasons (i.e. no strong Fighting Coverage in a mostly-Water, Ice, and token Ghost/Dark movepool) whilst giving it to the other two starters who have little gameplay use OR flavor claim to it comparably (a Football player and a drummer vs a Sniper).

 

[AztecCroc]

Venusaur never having the move

Venusaur doesn't have hands to make the blast with.

then we hit Galar where Cinderace and Rillaboom can learn it,

Cinderace is pretty Fighting adjacent, just look at its level up moveset. Also Cinderace is all about making balls of energy, that's literally its signature move and its GMax form's gimmick, a big fireball. Rillaboom gets it because big monkey.

but not Inteleon, the Sniper that has to be able to focus on a target so precisely they can shoot with centimeter precision at a great distance

That would be why it doesn't get Focus Miss yes.