When is the appropriate time to claim SL?
That is a tricky question because shoulders slant for most people and so some people will claim it when the hair touches the part of the shoulder closest to the neck (right where neck-length ends) and others wait till the hair is at collar bone where no one can deny it is truly at shoulder-length--usually referred to as Full SL. (See image below) I personally wait to get to Full SL to claim it but no one should be afraid to claim SL when it makes sense to them. Just know that you may be at SL for a long time if you claim it too early, which can be discouraging.
I agree with the statement that distance between shoulder and armpit being great and part of the reason for the "difficulty" in getting from SL to APL.
Carissa sort of touched on the other reason I think it takes long. Before SL, your hair is "safe" from friction caused by rubbing on your clothes. So even if you wear your hair out all the time, the only obstacle to retention is the drying air if you're not sealing (and that is assuming you are practicing good hair care and low manipulation).
Once your hair gets to shoulders, it's now long enough to rub against your clothes. So besides the drying air being the first adversary, there's now friction from your clothes wrecking havoc on your ends thus thwarting retention. So unless you're sealing and protective styling, then APL will just be a rumor you hear about for a long time, not a reality.
Someone may say that from APL to BSB or MBL or WL didn't take as long, but besides the distances being shorter, hair at SL has its ends actually hitting an obstacle. Beyond APL, hair/the ends lie(s) parallel to your body, almost cradled by your back or chest like a baby. As SL, your hair/the ends stop perpendicular to a part of your body almost as if they're hitting a wall. Then you turn your head about and it's like your ends are getting filed away.