I am definitely one of those people who hasn't played past gen 1. I can tell you that is isn't a snobby thing for me. It is simply that I became so quickly overwhelmed that I gave up. I fell in love with Generation1, I knew every pokemon, attacks, evolutions, the whole bit. I played Red and Yellow and loved them. I also watched the TV show religiously. When gen 2 came out I started to learn it, I was not expecting more, but I excepted that there was a whole new batch to learn and explore. But before I had the chance to settle into gen 2, gen 3 came out. I felt slightly overwhelmed and tried to get going on learning 2 so I could learn 3, and they slapped 4 on the table. At that point I just threw my hands up and said I'm out. It was too much all at once.
Now, lot of people have said to me 'you don't need t know them all to play'. Which is true, but when I get pokemon that I don't know what type they are, what kinds of attacks, etc etc, then I feel out of place and uneasy. I don't want to feel that way when I'm playing a game that is largely just for fun. Turn based play is slow paced and relaxed. I don't want to be stressed.