Not sure entirely, but probably because he seemed like a fresh face and few yet knew just how inept he was at campaigning. His huge lead seems surprising to us today, but the electorate was much more flexible back then. The Obama-Romney polls in the year leading up to election day 2012 were never a world away from the actual outcome.
I agree with the electorate being more flexible. I believe the 2004 election result (286-252) for Bush would have been a much, much larger victory had it been 1984 or 1996 for example.