Cause of load on the programmers’ part. They already coded the AI for NPCs with special behavior, it would be a lot of work to go back and re-code them to be fully playable and make sure everything worked, as well as avoiding potential bugs when their AT self is also out. That’s how I see it at least
In terms of programming, they probably wouldn’t reuse the AT coding at all for the playable version and would instead use other playable characters with a base, since only the model and animations could really be reused (but they likely wouldn’t be reused, since some ATs’ models and animations aren’t up to par for a playable character.) Thus, the workload would be about the same as for any other unique character, so someone can‘t really make much of a point that they would save huge amounts of work by promoting AT’s.
Besides, I don’t see how the code would conflict with itself if there were multiple instances of the same code running at the same time. After all, you could spawn tons of the same AT using the Isabelle glitch and the game wouldn’t glitch out (although it could crash from lagging out.) Since it would be confusing to players to have them both present, though, they would likely simply be removed from the AT spawn pool if the playable version were present.
I regardless still don’t think it’s likely at all that they’d promote AT’s since their priorities seem to be elsewhere. I’m just saying that there aren’t technical issues in the way, if for some reason they did want to do it.
Edit: Partial greninja’d