Because they are the Pokemons who ever ever came out first ^^ And from those 151 Pokemons only we moved ahead!
Although,i think no one hates the other generation Pokemons,its just the first gen pokemons are the most liked ones!
And if they don't love the new games then idk and if they refuse to play then they should give it a try!
Of course,first gen will always be awesome but the new ones should be checked out too!

I see.....

Many people i've talked to are just getting sick of the series. It has dragged on way too long, it's getting too repetative and all they've done is complicate it. The newer games are terrible compared to the older ones and they're also easier. Pokemon needs to end.....soon, or you'll end up with another DBZ kind of issue.

(sorry to rant, but pokemon is starting to get on my nerves with all the new games. They always say "Gotta' Catch em' all!" but how are ya supposed to do that when they keep adding more?!)

Because of a little thing called nostalgia.
And because when they were all younger they had better imaginations and felt closer to the Pokemon. I'm almost 18 and I started with the first 150 but I still love every new Pokemon that comes out because my imagination is still very strong. I see every Pokemon as very unique. But if I had to choose a Pokemon generation that was the best it would have to be the 3rd generation because it in my own opinion had the coolest Pokemon! Contrary to what Axel said, I believe that each Pokemon game is an improvement of the last story wise, but not Pokemon wise because every Pokemon is unique in his own way!