I think a lot of people associate it with their childhood, and they assume that just because they enjoyed it then, it isn't still enjoyable. I kind of understand that, I mean... if I were to watch most shows that I watched when I was five, I would be bored stiff, or be wondering how I found it good, and the same with most games I played then, but with Pokemon there's actually a lot more depth than people assume. I think a lot of people try to be all mature and stuff, but I re-introduced my friends to it by mentioning it every so often, and I've started rewatching the whole series - it's a lot better than I remember from when I was younger!
I guess it's the same as anything, people don't like it because of some pre-concieved ideas from before they even play or watch it. Sure, it's not realistic, sure it's still got sprite graphics instead of CoD or whatever where you can see a lot more detail, but that's what makes it so wonderful - the pure innocence of running around catching cute little things, even if there is violence in it, the worst you can really do is make a pokemon 'faint'. It's designed for kids but it's enjoyable for everyone - some people want something that's 18+ just so that they can feel cool playing it.
That's my opinion anyways.