I think it's mostly a matter of bolt-on necks being used by the more popular brands. I was once suspicious of bolt-ons and my first decent bass was a Carvin neck-through.
But now my various set-necks and through-necks are gone, replaced by Fenders.
It's the overall feel, and sound, you're buying, not some feature that appears to be "better".
Prior to Fender, very few guitars used bolt-on necks, and I remember they were disdained by the Gibson guys, but Leo made 'em work. Anyhow, the trend in guitars (Taylor for example) is to use bolt-on necks that resemble the traditional set-necks.
They are easier to manufacture, easier to adjust, and easier to replace, and at this point in history bolt-on construction is pretty mature technology.
And, before the advent of CNC machines, set-necks required a lot more expensive hand work by artisans.
Another way of looking at it is that set-necks are offered by Gibson, who has a long history of sticking a long-scale neck on a guitar and calling it a bass (not exactly cutting-edge R&D, ya know, but a lot of people like 'em) and Rickenbacker, who have stuck with a bass that doesn't have a sound that the majority of bass players are looking for. Through-necks tend to come from companies that are not as popular, or enormously more expensive (Carvin and Alembic spring to mind).
Blame it all on Leo for figuring out a reliable, inexpensive way to attach a neck.
