Gen 5
Starters, Watchog, elemental monkeys, Zebstrika, Leavanny, Swanna, Sawsbuck, Alomomola, Eelektross, Meloetta (31)
Gen 6
Starters, Vivillon, Pyroar, Florges, Gogoat, Hoopa (20)
Gen 7
Gumshoos, Crabominable, Oricorio, Minior, Komala, Bruxish (8)
(also Furfrou from 6 and Pikipek etc from 7)
So some interesting things with that. The 2 gens of starters aren't a surprise, starter releases are difficult for a number of reasons. Vivillon, Florges, Furfrou, Sawsbuck, Oricorio, Minior, and Meloetta are all mons with significant alternate forms that I could see being a problem to program(I'm sure Smeargle will remain Dexited forever for the same reason). And Komala has the potential to be broken as hell. That leaves:
Watchog, Elemental Monkeys, Zebstrika, Leavanny, Swanna, Alomomola, Eelektross, Pyroar, Gogoat, Hoopa, Gumshoos, Crabominable, Bruxish, Toucannon. Some of those feel like just them not bothering to include the mon*, because it's not popular and we have 10 things that do the same thing already. Eelektross and Alomomola feel like GF actively trying to screw over stall. Hoopa(form-shifting legendary with 2 signature moves) and Gogoat(maybe they've been trying to figure out ride mons and don't want to introduce Gogoat without that) are ones that can theoretically be justified.**
The monkeys, though...the monkeys confuse me. They were EVERYWHERE. They were gym leader mons, restaurant battles used them, every new gimmick format or odd trainer pulled them out, they were as bad as Zubat. And then they didn't show up in Alola, and haven't been back since. Either GF figured out their gimmick is boring, which seems unlikely, or something strange is going on.
*Bruxish, Swanna, Zebstrika, Watchog, Leavanny, Gumshoos, Crabominable, Pyroar, Toucanon. Like, if you told me GF straight-up forgot these mons exist, I'd believe you.
**I'm not saying that IS the reason, just that I could understand if it was the reason.