I don't know the history of A 36, but reasons to put in a maximum strength include ductility (higher-strength material may not be as ductile) and use of the "strong column/weak beam" design philosophy where you want to be sure that some elements fail before others.

Maybe by the time A 572 came along, they were willing to let elongation take care of the ductility problem, and if you want to make sure something fails before something else, then you use A 36 instead of A 572 to get a weaker member so there was no point in limiting the strength.

Hg