Why aren't AFCI breakers required for bathrooms/lavatories?
We're reading tea leaves here to guess at NFPA's intent.   NFPA writes the "model electrical code" which they offer for anyone in the world to adopt as their law. 

But politically, NFPA has been having a big problem.  Normally NEC changes are fairly trivial in cost: Pull a neutral wire on switch loops, gosh, you're using the /3 Romex instead of the /2.  It's maybe $3 per house.  Those increments have been getting bigger, as they've started so snake GFCIs and AFCIs in more and more places.  

Mind you, AFCIs started out as a solution for electric blankets starting fires in bedding.  However, they discovered that in actuality, AFCIs were catching arc faults caused by backstab connections.  Backstabs are a builder favorite (jab a wire in a hole and you're done, instead of having to form a shepherd's hook and torque to spec, times  say 200 connections in a typical house). Builders don't want to quit backstabs. (if they fail after closing, who cares?) 

Well, in 2014, NFPA was hellbound and determined to require AFCI or GFCI breakers on darn near everything.  And a lot of people were thinking "Why not just outlaw backstabs, then?"  Anyway, NEC 2014 was adding six hundred bucks to the cost of a house, which was actually going to have a material effect on real estate prices. 

Builders were saying "Oh hell no", and telling their Congresspeople to say "Oh hell no". And NFPA had to lobby right back, and influence public opinion. So states adopted NEC 2014 very reluctantly (seven still haven't), and some states still adopted it with some AFCI/GFCI requirements left out. 

Anyway, requiring both AFCI and GFCI on a circuit was politically "a bridge too far".