Rules-wise, one can't mix infantry and battle armor (including PA(L)s).
In-universe, PA(L)s aren't very common, with only few options compared to heavier BA designs, and outfitting large numbers of infantry with PA(L)s is expensive and logistically problematic. While affording all those suits is less of an issue, maintaining them is, do you train every single infantry man fixing their own suit? Logically, basically all factions figure it is better to get limited amount of specialized but far better battle armor than equip basic infantry with powered armor. PA(L)s are left for specops types, and even then most factions don't seem to bother.
EDIT Also transporting infantry equipped with up to 400+kg of equipment is far more difficult than infantry with just 40kgish of equipment.