Because they have never ever had one of their candidates win in a general election!
What's that? You mean that they haven't been around for a whole election cycle, and thus haven't had the chance to prove themselves, so it's unfair to judge them so early?
Come on now folks, seriously, you can't use the extreme length of the American election season as an argument that the Tea Party is a failure.
Reasons to forget about the Tea Party - they're such big budget cutters that they will never get any votes, and they're not really big budget cutters so hardcore minarchists should ignore them.
People are just tired of hearing of the Tea Party because they've been around so long (about a hundredth as long as the major parties!) so they are thinking of reasons to put them down. That is my theory. Otherwise I could never explain how people are so certain that a brand-new party is going exactly nowhere.