This is because "pure" +1 or +2 stat tomes have become very rare, so the increased AH prices represent their reduced ingame availability.
I have quite a few ability tomes on one of my pack mules for my many alts. The "library" used to consist of mostly +1 and +2 tomes for a long time, but now it's mostly upgrade tomes (+1 to +2, +2 to +3 etc.). So not only did the drop rate for "pure" stat tomes decrease, they are also required if you want to use the tomes that do drop.