The term master bedroom actually only dates back to the 1970's (or the very largest catalog houses of the 60's).

Okay so... residential architecture is sort of a hobby of mine. I've got a couple bookshelves full of historical plans dating back to Geo Barber, Sears, Gordon Van Tine, Shoppell's, etc.

So, standardized residential architecture became a thing in the 1800's with the advent of steam powered lumber mills. In Victorian and Queen Anne era plans, firstly, the vernacular was different. Bedrooms were called chambers, and while there was OCCASIONALLY a principal chamber, in most house designs the main distinction was between chambers for family, and (in larger houses) the maid's room (typically just one in the sorts of designs that made it into design books).

In the war-era designs, houses were frankly pretty damn small and there tended to be only one bathroom. One bathroom had been the norm basically since internal plumbing, and typically stacked over the kitchen to concentrate everything in one wet wall. It wasn't until the late 50's ranch and the 60's split ranch that you really consistently started seeing houses with a second bath, as architects finally acknowledged that people were starting to do home remodeling projects and might actually need more than one.

You didn't really see the master suite concept appear until the 1970's, or the largest ranches of the 60's. But not a lot of houses were built in the 70's due to economic problems, and those that were built tended to be split-foyer-garage type. When housing construction picked back up in the late 80's, the McMansions appeared. In the McMansion, the master suite was a main feature.