Because he can't win and people don't like throwing away their votes on ideology. First-past-the-post voting systems all but guarantee that when you vote for a

Because he can't win and people don't like throwing away their votes on ideology. First-past-the-post voting systems all but guarantee that when you vote for a third-party candidate, you're only really helping the first-party candidate most unlike the one you voted for (which is how we got Bush over Gore in 2000).