The Clinton machine built him up ...
Seriously? You don't think the GOP brought this on themselves?
The GOP is pandering only to the most right wing of its base. Anyone who advocates compromising or violating the sacred tenets (tax cuts for the rich, more military spending, etc.) is vilified as "RINO" - republican in name only - like it's a fundamentalist religion.
Middle & lower class Republicans see themselves treading water at best. Congress is doing nothing for them and the GOP presidential candidates are more of the same.
Of course Trump as new & different is going to get all the attention as well as the "he can't be any worse than what we've got" vote.
So the media takes it easy on Trump early on. First of all the media is looking for quick & easy stories that don't require time, money, or research. Second, Trump is more entertaining so the ratings are up.
The real research on Trump probably didn't really start until he had the nomination and was polling close to Clinton. Then people finally think " huh, I wonder what this guys really thinks and does since he might actually win."