Simply because the propagator of those types of threads feels truly vindicated, all full of piss and vinegar and ready to show everyone that they're right, even if it's just one loss among many wins.
The bitching threads when we win are very thin, dismissed, and laughed out of town.
Simply put, bitching threads after a win may make little sense, but wouldn't you rather see those than the 8 threads rubdawg posts after a loss, after he's already started 8 other bitching threads during the game?