I would think because some boats have a larger mainsail area that's effort is located midships enough to give best balance under main alone. A boat with a larger SA/D will simply sail better, or in less wind, with limited sail area. The slippery hulls of newer designs with short deep keels and detached rudders may simply respond better to main alone sailing.

I know your last part is tongue in cheek, but that may have something to do with it as well. Sailing under main alone may be too tedious for some to spend much time with.