I think it's obvious. It's easier to debut as a heel, and I'll tell you why. Almost every single heel will get booed. It doesn't matter how good of an act they put on or if they can wrestle, if a wrestler says something wrong about fans, that city, or a face superstar, they will almost always get booed. And getting booed is the goal, right? Heels will always get booed, always. It won't ever change.
On the other hand, it's much harder for faces to be accepted by the fans. Faces, unlike heels, have to actually prove themselves. If they don't perform well, hit their spots, and interact with the crowd, they won't be accepted. Fans won't cheer, chant their name, or buy merchandice, making this entertainer a failure. Fans like a guy who gets them excited, kicks ass, and is fun to watch. This is also why some heels get cheered more than they get booed.
In the end, it comes down to what's easier to do- piss somebody off or make somebody like you. That's why heels are easier to debut than faces.