I wouldn’t be surprised if one factor considered is that a headstock TR adjustment requires removal of some wood and might even weaken the neck joint. It made more sense, simpler and effective, to put adjustment at the heel, like a Fender. But, unlike a Fender, Alembic made the TR adjustment easily accessible. OTOH I could be completely off-base, it’s just my guess.
I find most headstock TRs also easy to adjust, so long as one has the right tool. I have sets of TR wrenchs.
Bill, tgo