IME, modern compact road frames were an attempt to use marketspeak to justify a smaller run of frame sizes. After a year or two it became apparent that mfrs needed more than just 3 or 4 frame sizes regardless of top tube angle. At that point it was too late to abandon the marketspeak justification for the new/improved design, so they kept on keeping on with the marketspeak and just as many different frame sizes. I'm talking about the modern inception of this design, now 20yrs ago.
I haven’t done a complete survey of
every bicycle manufacturer since the beginning of time, but your assumption is flawed. Trek has consistently offered 5 to 9 frame sizes since the 80s. The earliest years have fewer frame sizes then the later years. Cannondale in 1984 offered 4 frame sizes in road bikes. Today, they offer 6. They don’t use a radical slope but they offer more frame sizes. The Specialized Allez was offered in 6 sizes in 1991. Today they offer 7 road bike sizes with a fairly good slope to the top tube. I’ve bought a lot of Specialized mountain bikes over the years and not one has been selected from only 3 or 4 frame sizes.