I think you have to go back in history, at least in the U.S., hi-fi was larger speakers, often adapted from large high efficiency speakers--think of JBL or Altec, Bozak, EV, or kits of drivers/crossovers that required the end-user to build a cabinet. Ed Villchur developed the acoustic suspension speaker, which was meant to deliver bass in a smaller cabinet (with the drawback of lower efficiency), but the design allowed for ease of placement in the "living room" without dominating the space.
I have not researched the first use of the term "bookshelf" for speakers but suspect it coincides with this development. Obviously, there are also ported speakers that are smaller, but the acoustic suspension speaker, originated by AR, followed by KLH and then Advent (Henry Kloss being part of all this) set the pattern in the U.S. Most people did not want the intrusiveness of big hi-fi in their home. I base this on my experience slinging hi-fi back in the era, circa 1969-72.
Once the "high end" took hold, which was really a small segment-- KLH 9, Infinity Servo-Statik, the Wilson Wamm (original), the early Magneplanar, all concerns about size seemed to be irrelevant. AR even got into the act with the LST, which was a pretty cool speaker, but a beast--and notoriously inefficient.
My perspective. What was going on in the UK may be a different story. I bought my first pair of Quads in 1974. Sort of British industrial-- not exactly something that blended into the decor of the "lounge" but there it is. The BBC monitors have a different origin, don’t they?