Gen 1 was the original generation, so there were no earlier Pokemon to add evolutions to.
In Gen 2, they added Crobat, Pichu, Cleffa, Igglybuff, Bellosom, Politoed, Espeon, Umbreon, Slowking, Steelix, Scizor, Kingdra, Porygon2, Tyrogue, Hitmontop, Smoochum, Elekid, Magby, and Blissey.
In Gen 3, they added Azurill and Wynaut.
In Gen 4, they added Budew, Roserade, Mismagius, Honchkrow, Bonsly, Mime Jr, Happiny, Munchlax, Mantyke, Weavile, Magnezone, Lickilicky, Rhyperior, Tangrowth, Electivire, Magmortar, Togekiss, Yanmega, Leafeon, Glaceon, Gliscor, Mamoswine, Porygon-Z, Gallade, Probopass, Dusknoir, and Froslass.
In Gen 5, they made a region with no old Pokemon in it. However, this is hardly a reason not to add more evolutions, because Diamond and Pearl managed to add several that could only be obtained after obtaining the national Pokedex.
I thought that they were going to add continuations every even generation, but in Gen 6 they only added Sylveon. I was rather disappointed. Why aren't they doing this anymore? Is it because Mega forms are simply "in"?