ORIGINAL: MikeL
It's the legacy of tiny displays that were limited in the number of characters they could display.
That explains why they chose four letter mode names, but not why they chose ACRO over AIRP, PLAN, ARPL, AERO, etc. The OP was asking why they chose ACRO and what is stands for. So far, it sounds like it is short for "acrobatic", and they just like it better than the other 4 letter mode names I suggested here.