Indeed central vacs are pretty uncommon. My architecture teacher actually discussed that fact. He said they were probably the best innovative major appliance for a home. "and you dont need to drag out your eurE!ka" as he said! Best teacher in my school! He said out of the dozens of houses he has built for clients, most didn't really want a Central vacuum.
My grandparents house was quite surprising to me, for them to not have a central vacuum. The house was one of the most expensive, and high end houses when it was built in 1915, for Tom Dennison, who was definitely not short on change, if you know what i mean.
The shear size of the house is what is bizarre about it though. Many other mansions for the time had them.