Hi Greg;
There are several reasons.
1.) Too wide. Chunky, in fact.
2.) Just-plain-awful DA first shot trigger pull, and not capable of being easily fixed via H&K's "drop-in" Match trigger, which can't be fitted to the USP .45C.
3.) High bore axis, similar to SigSauers, therefore more muzzle flip than, say, a Glock 30.
4.) Steel magazine bodies have been reported to come apart at the weld, though this must be rare.
5.) Mags only hold eight rounds.
6.) Quality control has been a bit spotty with the USP .45C, it appears. Some "lemons" have turned up. I know, I had one that wouldn't hit consistently (poor precision).
7.) The HE ("Hostile Environment") slide finish isn't as durable as it might be, and shows wear relatively quickly at high points.
Those are the negatives, and for me, they were compelling enough that I recently got rid of my two USP .45Cs.
Besides being a longtime H&K fan, I got them originally because they could be had in a form permitting "cocked and locked" carry in a left-handed .45ACP without requirement for an ambidextrous safety. I feel the Glock 30 is overall-superior to the USP .45C, as long as it fits your hands. If not, well, the slimmer grip size is one of the "positives" of the USP .45C...