I sometimes revisit the OP's original statement / comment / question and come to a different conclusion than my first reply, but not this time.
Why don't more builders use fanned frets / multi-scale?
Dollars, nothing but dollars. If a Jaco, Flea, Trujillo, Sting, etc comes along and is playing a fanned fret bass, builders might move that way, but they aren't going to until it makes $$$.
With string making technology solving the floppy B string problem for most, it's even less likely for builders to make the change to a fan fret model now than it was in the past.
It's not at all uncommon for the first solution to a problem to be a decent solution, but maybe not the most economical solution. Economics usually wins in the end, so the string solution to a floppy B is the most widely implemented solution. Is a fanned fret bass better than a straight fret bass with balanced strings? I have no idea, but I can try balanced strings much easier and cheaper than a new bass, and if it solves what I thought was a problem, I'm no longer looking for a solution.
Why don't more builders use fanned frets / multi-scale?
Dollars, nothing but dollars. If a Jaco, Flea, Trujillo, Sting, etc comes along and is playing a fanned fret bass, builders might move that way, but they aren't going to until it makes $$$.
With string making technology solving the floppy B string problem for most, it's even less likely for builders to make the change to a fan fret model now than it was in the past.
It's not at all uncommon for the first solution to a problem to be a decent solution, but maybe not the most economical solution. Economics usually wins in the end, so the string solution to a floppy B is the most widely implemented solution. Is a fanned fret bass better than a straight fret bass with balanced strings? I have no idea, but I can try balanced strings much easier and cheaper than a new bass, and if it solves what I thought was a problem, I'm no longer looking for a solution.