Why are so many Pokemon from Gens 2 and 4 either not available or just not really useful to use in-game? When I take a look at these Pokemon, I see so many that are just awful.

Compare Gen 5. Where the Pokemon that people generally don't recommended for use boil down to
>Pidove line
>Basculin
>Maractus

Of course, Gen 5 was designed to stand on its own two feet a bit more, but that doesn't mean out Gen 2 and 4 selections didn't have a lot of crap.