AT&T should really be considering offering more HTC phones and other devices on their network. They are good quality phones and people tend to like them a lot. I think there are a few more planned for later this year or next year.
Many of us hoped the selection of HTC phones would improve with the loss of iPhone exclusivity. But it seems that AT&T is still putting most of their eggs in the iPhone basket. Plus, they also maintain a decent selection of Samsung and LG phones. Those brands aren't my thing, but that's just me, and plenty of people seem to like them. I suspect these are the main reasons we don't see many HTC phones on AT&T.
If you look at the other carriers, they tend to put on emphasis on one manufacturer or another, and don't carry a huge selection of all brands, just as a matter of practicality. T-Mobile banked heavily on Android and HTC as kind of counter-programming against the AT&T iPhone exclusivity. Verizon did a similar thing, pushing Motorola in a big way (and carrying some HTC) before they also got the iPhone.
In the end, the cell carriers have finite room in their handset lineups, and have to pick and choose what brands and models they want to sell. Unfortunately for us, AT&T has not decided to invest heavily in HTC. I think the only decent HTC phone coming out on AT&T for the rest of the year is the Holiday. Also the HTC Puccini is coming to AT&T, and has the potential to be a really good tablet. But of course, that is not a phone.