Y'know, it's not even the fact that it's limited to 150 pokemon that's the problem, Kanto and Unova also only have 150 mons and their regional dexes are fine.
It's also not that the mons are too rare, maybe this is a hot take but having some mons be incredibly rare makes the process of actually catching them all feel like a more significant task.
But man... why did they have to handle the type distribution so fucking poorly? It's not even just fire, either. Other types like electric, ice, ghost, dark, etc are all hard up for dex slots. Meanwhile types like Water, Flying, Normal, and Bug are utterly drowning in dex slots. Especially water, like WHY did gamefreak have to include 3 water/ground lines? And 3 water/Flying lines? No reason. I understand that some types are rare and others aren't, but the fact that there are more water/ground and Water/flying dual types than there are fire or ice types is utterly retarded.