Why cement under upstairs bathroom floor?
It was (and still is) common to float cement on top of the floor for installation of ceramic tile. It provides a level and firm substrate to which tile mortar (thin-set) will adhere. 1 1/2" seems a bit thick though, maybe there were irregularities with the floor which had to be compensated for. I try not to float so thick unless I have to, and now I use cementitious tile backer board when appropriate. Or sometimes I mortar the tile directly to a smooth firm plywood subfloor in new construction applications.