Could it be that HTC have started performing badly as a company, so Apple don't consider them worth the lawyer fees any more?
That and a few more. I imagine it to be a mixture of several factors:
1. HTC successfully managed to transfer cases to a venue away from Californian courts
2. ITC found only one of the four / five Apple's patents to be valid and infringed by HTC devices (one related to data-tapping, i.e., detecting links, phone number etc within emails and texts)
3. HTC won a case in Europe on similar patents, where most of Apple's patents were deemed invalid. Only one was deemed valid, and HTC was found not to infringe that.
4. Judges in Delaware found that few of HTC standard essential patents to be likely valid and possibly infringed by Apple and HTC exploring seeking an injunction against iPhone 5 and new iPads using these patents.
So, Apple saw a possibility of loosing the case.
It is still significant development coming from Apple. They have repeatedly held that they are not in the business of licensing "product differentiating features". Only known licensing deal is with Microsoft and Nokia. HTC is the first company which also manufactures Android phones to have reached a settlement with Apple.