There were two major factors. The lesser factor was it was just freakin' new, and really Windows 7 was Vista SP1, or as my father called it - "Vista without the suck." There were lots of small, minor tweaks that needed to be made, and yes of course driver support was a nightmare for a while.
But to me, the biggest stumbling block of vista is hardware did not keep up, or rather it jumped way ahead of the hardware curve. There were tons of computers selling at the vista release date whose specs were fine for XP that microsoft bullied the OEMs to switch to vista that absolutely had no business running it. A laptop rockin' a single core celeron and a gig of DDR is perfectly acceptable economy/low grade machine for XP... not so much for Vista/7. Yet it was tons of machines just like this which were what vista came on for a year or more. It took them a long time to figure out vista/7 needs... NEEDS.. 2 gigs at LEAST of ram to run (and preferably 4 or more). By the time 7 launched, the economy model price point machine specs had caught up with the OS's requirements, so it got the better reputation.
That and I still hate the changes to the control panel structure. F^%$'in Network and Sharing Center.