For me it's because most-all of the Hoenn/Sinnoh pokemon look so outlandish and in several cases silly. Some of them make me wonder if Pokemon Co. ran out of ideas (QED- Bidoof). The thing I liked about Kanto and Johto (and a decent number of Hoenn) pokemon is that they actually looked natural and organic. I don't need flaming-butt monkeys (or is it flaming butt-monkeys?) to make things more interesting, and in fact I would have been just fine if they only added about fifty or so new pokemon past two-hundred fifty-one. It would give them room to make some new, cool additions without having to pull things out of their asses to fill in the gaps. But that's just my two cents, and my party has always been retro in each game (with the recent exception of Absol overthrowing Pikachu) so I'm a tad biased.