I can understand it in Dreepy's case as it's basically made to be unusable early-game, but the Jangmo-o case has actually bothered me a bit since the release of Sun and Moon.
I mean, it's not like using the early stage of a pseudo-legendary is going to break the game or anything; in fact the opposite, as with a few exceptions they are often underwhelming choices for the story early-on...
Nah, pseudo-legendaries are much more devious than that. The first stage tends to be pretty good early on, as it tends to learn a couple of decently powerful STAB moves to use off decent stats for that point of the game. Moves like Dragon Breath or Rock Slide at level 15 is nothing to sneeze at.
However, the
middle stage tends to be hot trash. Moves that were awesome at level 20 are not so by level 40, and by 50 you're begging for the mediocre period to end. This is also the point where you realize how much XP these things can soak up without gaining a level. And how slow and frail they are, so even if you put them at the front of your party so they gain more XP, they tend to get KO'd by random stuff and don't get any XP at all (thus falling further behind the rest of your team ...) They truly resemble one of those "monkey's paw" traps, where you are enticed by something that looks good, but by reaching for it you enter a situation you cannot escape without letting go of the prize. The prospect of getting a powerful Tyranitar to unleash upon your enemies is too enticing to give up, but as anybody that has trained a Pupitar through the level 40's in-story can tell you, you really shouldn't let that prospect blind you. And at the end, joke's on you, you have your pseudo-legendary, but there are like five (to be precise,
exactly five) trainers left for them to fight before the whole game is over. And at least a couple of them specialize in typings your pseudo-legendary is weak to. All that suffering for so little.
Anyway, regarding the Jangmo-o situation, I think the designers were faced with a dilemma, namely which of these two questions they wanted to guide the design:
1) What roles do we want Jangmo-o to be able to fit?
2) What role do we want Jangmo-o to fit
in this game?
As
ScraftyIsTheBest outlined, most pseudo-legendaries are built so that you find them somewhat underleveled for the area where they are encountered, and have to use the base form for a handful of levels before they evolve, then use the middle form for a while until you get your "high-investment-high-reward" prize, the final evolution. However, with the modern XP formula, the impact of presenting a Pokémon severely underleveled is dampened quite a bit, as it is much easier to bring up to par than what used to be the case. This has led the designers to try presenting the base form at a level appropriate to the area, and crank evolution levels up to match. Hence Deino, which evolves absurdly late at level 50 and then again at
64. Dreepy is another example of this school of thought.
However, while this makes the Pokémon suitable in that one game, it makes it much more difficult to fit into a later game. After their debut generation where they play the role of super-elusive late-game prize, pseudo-legendaries tend to be re-assigned to serve as rare Pokémon encounterable somewhere earlier in the game. Pokémon you can pick up around the third Gym and use in-game throughout your adventure, in anticipation of that sweet pseudo-legendary that will make it all worth it (in a similar fashion to how cuckoo eggs look enticing to prospective bird parents).
However, Deino and Dreepy are completely
pants for this role. Their late evolution makes them awfully underpowered for most of your adventure, and depending on the game you may not even see them evolve twice before the credits. They were designed as "prize Pokémon" appropriate for the level they are found in their first game, which makes them unsuitable in any other context. They simply remain too weak for too long. Pokémon like Gabite, Metang, or Pupitar can at least pull their weight in the early mid-game, and their final evolution happens around Victory Road or thereabout.
Going back to Jangmo-o, I think the designers looked forward when they designed it. They wanted it to not be useless in subsequent games, which means "midgame" evolution levels, but they also wanted it to be exclusive to the final area, which for all practical purposes means "endgame" encounter levels (if they had presented it at level 15, it wouldn't take more than a few battles for it to reach the level for evolving into Hakamo-o anyway, thanks to the XP scaling formula). It meant that immediate Hakamo-o was inevitable, and so was almost-immediate Kommo-o.
What baffles me is how they not only reverted to the other school of thought with Dreepy, but somehow made it even worse. It evolves from a weak base form at level 50, but it is also encountered at level 50, meaning that the weakling Dreepy stage is completely irrelevant in its debut game, but it completely torpedoes its viability in all future games. Dreepy is pitifully weak until level 50, making it a millstone for the player's team unless it is obtained at a similar level, and even when you get Drakloak it will already be underpowered for a level 50 'mon and you're stuck with it for ten more levels, which may be more levels than the story mode has content for.
I suspect that future games will just give us Drakloak directly at level 30-something, to make it a somewhat viable companion for the story mode. It's the level at which Dreepy should have evolved already, so there could be some overlap between the weakness of the first stage and the "viability power curve" of the game. Dreepy somehow has the worst of both worlds. It evolves way too late, and is way too weak until it does so, and it
still didn't even get a satisfactory progression in its debut game to show for it.
