Because most third party voters know perfectly well that their candidate is not going to win the Presidency (or any other high Federal office). By definition, that makes third parties protest votes for people who know that Crook/Moron (D) or Moron/Crook (R) is going to be President, but who can't bring themselves to bother voting for either.
The bottom line remains that Bush I, Dole, McCain, and Romney can go on pretending that third party candidates cost them the election, when the fact remains that people wouldn't be voting for those third party candidates (or staying home) if the GOP fielded halfway decent candidates to begin with.
Incidentally, there are third party breakaways on the Left as well: Ralph Nader and his Greens. Gore blamed them for his loss in 2000. Chances are, most of those people wouldn't have voted for Gore either, even if there were no Nader and no Green party.
That attitude serves ONLY to accelerate the decline and destruction of America.
It does NOTHING toward sending any entrenched establishment a message.