I think all the reasons why G&L basses aren't more popular are pretty much stated.
I didn't read all of the posts, so I hope that I'm not saying something that's already been said.
I feel that another reason why G&L basses aren't more popular is because they are Leo Fender trying to out do Leo Fender.
I mean like after Leonardo Di Vinci painted the Mona Lisa..............how could he do better?
After having invented the P Bass, the Jazz Bass, the Stingray...............how could Mr. Fender out do perfection?
So for me, G&L was always Leo Fender competing against Leo Fender. G&Ls were copies of the best basses already made.
G&L basses are excellent, well made instruments on par with USA Fenders and Musicman. But, they are copies of those basses.
I own a 1981 G&L L-1000. It's a wonderful bass! But, it's not better than a P Bass and that's more or less what a Wunkay is, a P Bass with 3 pickup selections and the one selection that would be like a P Bass (series with hum cancelling doesn't exsist) instead it's series with bass boast and it hums like a single coil because of a capacitor wired into one of the coils.
Many pre-lawsuit G&L basses have neck issues. And that bullet truss rod adjuster just begs to be stripped!
Also, in the beginning, G&L basses were like the nasty 70s Fenders and Musicmans. Three bolt neck and the bullet truss rod adjuster. Two very undesirable features!