"When did we ever let one bad game ruin a franchise?!"
Even though he's joking, JacksFilms makes an excellent point. For decades, the Tony Hawk Pro Skater franchise was a hugely successful franchise! I myself remember purchasing at least both undergrounds and the Gameboy Advance port of the original! So why now after one bad game has the series seemed to come to a definitive end?
Well in truth I don't think this was the first bad game as Tony Hawk: Ride was a pretty colossal failure. Even so Sonic The Hedgehog has continued to make awful game after awful game and still has about a game a year.
If you want the truth I think it's mostly apathy on Tony Hawk's part. The game was a low-budget mess, and I think about as much time went into development as anyone behind the scenes cared to give to it. If you want the humor watch the video below.