In all honesty there is posts here from staff about the situation but in short - ASUS needs to go into talks with each provider and then testing needs to occur etc
Delays are caused by how populour the brand/phone is amongst many other standards that need to be met.
Going by the updates in the year it seems more are being added as we go along so I assume at&t will help support the ROG2 in the future but I can't say for sure.
