Having (or not) AT-AT's in this game is secondary to a majority of people in deciding if theyre going to buy it (or not).
First its a ruleswise and willingness to play in a squad scale free mode (i.e. not using the skirmish mode of IA).
Second is a budget concern (maybe its a primary concern for some). Players already play some form of FFG Star Wars related game (X-Wing, Armada, IA) and they dont have the money to invest in a second or a third game (or fourth).
Third is a wait and see in how and by how much FFG are going to sell the expansions. We already know theres vehicles planned so I would bet that an AT-AT isnt a far fetched option (like the Epic scale in X-Wing an AT-AT doesnt need to be in the proper scale to make it work in Legion - could be in some special scenarios and shouldnt be allowed in tournaments for example).
Fourth... Its a business for FFG. Im sure they wont waste the oportunity to cash in a few more dollars by not releasing a vehicle thats a wet dream for most Star Wars fans. Ohhh... and we already know that the First Order will field their version of AT-AT's in The Last Jedi...
Of course there always some guys which will decide based on that but it will be a minority.
Edited by Kentares