Nobody sues if they can avoid it. Lawsuits are extremely expensive, and companies will exhaust all other options first. The lawsuit means Apple demanded more than HTC was willing to pay, and decided they would fair better in court.
Also, "iPhone knockoffs" doesn't necessarily mean patent infringements. Without doing in-depth research into the specific claims infringed, you can't say for sure whether any infringement took place.
Patents and patent law are incredibly complicated. There is no simple answer to _why_.
I'm not saying Apple should have won or that that case isn't in some ways relevant--only that it wasn't certain that they would lose.