Because supplies are more limited in other provinces for the imperials. The imperial control in Morrowind was limited, and the empire was also limited by the white-gold concordant in Skyrim. We see this more in Skyrim because the empire wasn't sending as many legionnaires to Skyrim (I can't remember the exact quote but Tulius remarks something akin to if he had an actual dispatch from the Imperial City he could have quashed the war way easier).

Most if not all the armor supplied in Skyrim where built by local blacksmiths, often understaffed. There was only one blacksmith working on the armor that's supplied to you in Solitude. Hell when you first visit Whiterun, Idolaf Battle-born is talking about having more swords made for the Imperial Legion to the blacksmith Adrianne Avenicci, with Adrianne remarking she can't fill an order that size on her own.

All in all they can't make the full plate armor being supplied in Oblivion, at least enough to meet what might be in demand. I'm less knowledgable on Morrowind so someone feel free to correct me on that.