OMimistasia said:
Don't know why he's gone.
Where do you get the impression that he's gone anywhere? There haven't been as many Mario sports games this generation, but he was still playable in both Mario Strikers Charged and Mario Super Sluggers (even featuring on the cover). He also earned a boss role in Brawl, and clearly influenced Dino Piranha's design in Galaxy.
He's missed a couple of games, sure. But there are actually some pretty logical explanations for most of them:
Mario Kart Wii: the character models in this game are of a lower quality than Double Dash's, which I can only assume is because they didn't want complex models to cause lag during 12-player online races. Petey is one of the most unorthodox characters in the series, and would look noticeably lower quality in such an unrefined state. Additionally, take a look at the character choices and you'll notice that many of them share the exact same foundations as others. Baby Mario/Baby Luigi, Baby Peach/Baby Daisy, Koopa Troopa/Dry Bones, Yoshi/Birdo, and perhaps most importanly, Bowser/Dry Bowser.
In conclusion, the reason Petey wasn't in this game is because they wanted a cast of characters that would cause the least amount of hassle. The likes of Dry Bowser and Funky are only in the game because they're little more than reskins of other characters.
Mario & Sonic Olympics (both): this should be pretty obvious, really. Given Petey's design it would be very difficult and probably quite arduous to accommodate his model in certain events. I do think he should at least have featured as a boss, however.
As for Dry Bowser becoming "the next Petey"...well, considering he wasn't playable in Mario Super Sluggers--a game that starred every Mario character under the sun--I somehow doubt it.
Personally, I think Petey is a brilliant character, so it's always a shame when he doesn't make an appearance. But we just have to accept the fact that with such an unorthodox design, he isn't always going to workable. I do, however, believe that he will continue to act as a recurring boss/playable character for the foreseeable future. It's become more than just a trend to put him in games.