HTC has been struggling in the past few years, and it looks like their struggles are continuing now. As pointed out by Wave7, both AT&T and T-Mobile have removed HTC from their websites. None of HTC's devices are listed there at all anymore, not even the HTC 10 which is not even a year old at this point. On AT&T's website, you want to refine the search of devices by manufacturer, you won't see HTC listed, which shows that these two carriers are looking to focus on more lucrative manufacturers like Samsung, Apple and LG. Additionally, while Verizon is technically selling two HTC devices, almost none of their stores actually have any in stock.
This comes just in the same week that HTC announced their earnings for the previous quarter, which showed that they were continuing to lose money and sell even less smartphones. HTC also announced during their earnings call that they are going to shift more of their focus from their low-end and mid-range devices back to the high-end. This is due to the fact that the high-end devices also have a higher profit margin, although there is also more competition in that area. HTC stated that they will launch 6 to 7 high-end smartphones this year, of course they have already announced two last month in the HTC U Play and U Ultra, and early reviews aren't looking to promising for the Taiwanese company.
HTC has seemingly been moving towards selling their smartphones unlocked, something that a lot of manufacturers are leaning towards lately. As they can sell directly to the customer, without having to work with carriers. This is good because it means that HTC can push out updates faster, as they don't need to be certified by the carrier, but this also means there are less stores carrying their devices. It also means that there are less employees pushing their new smartphones when they come out. For many customers, they head to their local carrier store for a new phone, and if it isn't available at T-Mobile, Sprint, AT&T or Verizon's store, it won't be in the discussion as their next smartphone. Unlocked smartphones are starting to gain some traction in the US, but it'll be a while before it becomes profitable for manufacturers like HTC, Huawei and even Sony - all of which sell their flagships in the US as an unlocked device.