The just-unveiled HTC 10 will unfortunately not be sold by AT&T in the US. It's still headed to the other three of the 'big four' carriers stateside in due course, however - so you'll soon be able to pick one up from Verizon, Sprint, or T-Mobile. And if you love AT&T and crave an HTC 10 at the same time, there's a choice for you too, albeit a rather expensive one. See, the unlocked model that HTC is selling through its own US online store is compatible with AT&T's network, so no one's stopping you from purchasing one for $699.99 outright and then using it on that particular carrier....
Source: http://www.gsmarena.com/at_t_wont_sell_the_htc_10_bell_gets_an_exclusive_in_canada-news-17739.php