Bandwagon effect.
Everybody loves supporting a winner, hates supporting a loser. Polls become self-fulfilling prophesies.
I think they'd get the same results if they asked: "Who do you think will win?" Like you say, people don't want to say their choice is an also-ran.
I think it would be interesting to ask:
1. Who is your choice for the GOP nominee?
2. Who do think actually would make the best president?
I haven’t supported the winner in the GOP primaries since 1984. Has not caused me any heartburn.....
Exactly what I was trying to say...put yourself down for a silver star :)