1) Well, the obvious answer is for gameplay reasons, but that’s probably not what you’re asking. I think a good starting point would be thinking about what ‘knowing’ a move actually entails. Do they just get these powers, have them, and then never have to practice them? I’d think not. I mean, that’s not how martial arts or sports work in the real world. You have to practice your skills constantly in order to be any good, and it’s not like you can just, like, practice firing a Flamethrower straight forward on level ground and be done with it; you want to be accurate from as many different positions and under as many different conditions as possible. Maybe five moves is just too much for most Pokémon to keep up that kind of skill level?
2) I think Ludicolo is actually sort of a bad example, because Pokémon that evolve using stones normally lose the ability to learn most moves by leveling, and that’s probably to do with the same ‘old dog, new tricks’ mentality that’s behind most evolved Pokémon learning the same moves but more slowly – the older forms are more set in their ways; they might learn new applications for skills they already have, but completely new abilities are just more complicated than they want to deal with. A more interesting example would be the transition from Lotad to Lombre, because Lombre actually continues to learn new moves, just different ones – Fake Out instead of Mist, Water Sport instead of Mega Drain, and so on. Obviously nothing physically prevents Lombre from being able to use the moves Lotad could learn, but it won’t learn them if you try to teach it, and I think maybe that’s because, specifically for Pokémon that work like this, learning the move is somehow tied to the mindset that comes with being smaller and weaker. Once they’ve got the skill they can maintain it by remembering how they felt when they learned it, but if they never learned it in the first place they just can’t see themselves needing it in the same way – like “sure, little kids have to learn to do that to protect themselves (or something) but I’m grown up now; why would I need that?”