That still offers very little explanation about what their motives could be. Do they just want to generate some money, or are there actual concerns about backwards compatibility?
Conveniently, they get to charge manufacturers for this certification process; and if consumers don't have a compatible phone, they get to advertise new expensive ones which come with installment plans.
Overall, it's a win-win for AT&T, and a loss for customers.
[0] https://www.theverge.com/2018/6/28/17515464/att-911-emergenc...
The higher your resolution is the more you can be sold for.