The HTC 10 was announced today, and it was also announced that Verizon, Sprint and T-Mobile would be carrying the device. Notably absent there is AT&T. The HTC 10 won't be available at AT&T stores, nor on their website. However, those that are looking to buy the HTC 10 and use it on AT&T can still do so. As the company is selling their new flagship device on their website as an unlocked edition and it will work on both T-Mobile and AT&T. In fact, HTC has the option there for you to choose which carrier you want. The only differences here between buying the unlocked version and one from the AT&T store is the bloatware and carrier branding. There will be none of that on this device - and after seeing what AT&T did to the LG G5, that's probably a good thing.
With the HTC 10 costing $699 full retail, that's quite a bit of cash to throw down at once. This is a reason why many prefer to buy phones from carriers, as they can split up the payments over the course of 24 months or so. However, HTC does allow you to do payments through PayPal Credit. So at least there is that going for HTC, and those that want to buy the HTC 10.
AT&T hasn't mentioned why they aren't carrying the HTC 10, but it does seem like they may have an exclusive smartphone on the way. Typically when one carrier doesn't carry a smartphone, it's either because the manufacturer's previous flagships haven't performed well, or because the two companies have an exclusive device that will be coming soon. Knowing AT&T, it could be either or. But it's most likely an exclusive. Now that's not confirmed nor set in stone, so take that as speculation.
If you are on AT&T, it looks like the only way you'll be able to get the HTC 10 is by buying it from HTC. And that might be the best way anyways. Especially if you don't want to deal with all the DirecTV bloatware we've seen on recent AT&T smartphones. The HTC 10 is available for pre-order today!