because venusaur, blastoise, charizard, nidoking, arcanine, poliwrath, alakazam, golem, gengar, starmie, mr. mime, scyther, electabuzz, gyarados, vaporeon, jolteon, aerodactyl, snorlax, articuno, zapdos, dragonite, and mewtwo are all really cool pokemon, as are their evolutionary lines (or at least imo). that's a pretty good amount of good pokemon from the region, and certainly more than gen II, gen III, and gen IV had
not to mention that gengar, starmie, gyarados, vaporeon, jolteon, and dragonite are all excellent battling pokemon
Don't get me wrong, I personally think Gen V was the best. But Gen I has a lot of good Pokemon that you can't discount.