There are a couple of things that make them a harder sell IMO:
1. Price - they cost more and price themselves out of some people's budgets entirely, and there are also some people that look at them and say "Gosh, I could buy two Glocks for the price of one H&K!" Yeah well, I could buy two Hi-Points for the price of one Glock. And? In other words, someone who buys an H&K usually knows ahead of time they want an H&K before they head to the store.
2. Magazine prices. For the USPc 9mm, .40 and .357 Sig it's usually not too much more than Glocks (these days who knows), for .45 ACP models, you're going to pay. That turns some people off.
3. DA/SA - and the safety. Some people don't like it. Some people don't know that LEM with no safety (or with) is an option instead of DA/SA. Some people don't like either one and prefer striker fired. Up to them.
4. On the USP and USPc specifically, the rail type can be a problem for some. The adapter is no big deal, but still it's something else to buy if someone intends to hang a standard interface light or laser on it.
5. A USPc 9mm will hold a couple less rounds than the similar sized Glock 19. Some people don't like that.
They will sell, but I would think they will definitely move slower than Glocks, M&P or XD, etc. Same reason the Mercedes dealer's lot doesn't have as many cars on display as the Honda dealer. There's just a whole lot more people that can afford a Honda and for whom it does everything they want, so no need for them to go upscale unless they are specifically looking for the things the Mercedes offers that the Honda doesn't.
If you have a hard time moving them you could try a couple things:
1. Lower the price a bit - I know, duh.
2. Offer a longer layaway period on them versus other brands to help take the sting out for some who really want it but might need longer to come up with the cash.
3. Put together a little package with some "extras" thrown in to sweeten the deal.
4. Go to the HKPro forum and offer up a group buy on all of them at a bit of a discount.
My quiz on if someone should buy an H&K:
1. Do you want a hammer fired pistol?
No - Okay. Buy a Glock, M&P, XD, Walther, Steyr or FNS.
Yes - Proceed...
2. Do you want an all metal hammer fired gun?
Yes - buy a SIG, CZ, 1911, Makarov, BHP, Baby Eagle, 3rd Gen Smith, etc.
No - Proceed...
3. Do you want the best polymer frame, hammer fired pistol on the planet?
No - 2nd place ain't so bad. Buy an FN.
Yes - Congratulations - you want an H&K. Will that be credit or debit?
As for me, I loves me some USP and USPc pistols. They are my favorite H&Ks.