Because he was 6 years removed from being both Governor AND Chairman of the RNC. In 2008, when he ran, he was the last GOP Governor of Virginia. He was a first-tier candidate.
The other first tier candidate who expressed an interest was RINO Congressman Tom Davis. However, Davis behaved like a petulant child when this race came up. He demanded the nomination be handed to him without opposition, which was a ludicrous demand given that he was not considered a Conservative. When he was informed he’d have to campaign for the nomination, he not only refused to run, he quit his Congressional seat in a fit of pique, which went to the Democrats.
Nobody thought Gilmore was a spectacular candidate, but did consider him credible. Had we known he’d perform like a second or third tier (desultory, as it turned out) joke, obviously we’d have been better off taking chances with the second tier candidate who’d have been willing to take a kamikaze run at Warner, that being Del. Bob Marshall (who lost by less than a single % point at the convention to Gilmore).
Didn't he do some robocall against Republicans when he was in Congress or something like that?
Well, to be fair, 2008 was just a bad year for Republicans.