Just a thought (if it’s already been put forward, sorry). Back in the late ‘70’s and ‘80’s and even on there was really no other option to generate that MM tone, and it set the bass world on its ear. With today’s almost unlimited pedal and amp choices, the Sting Ray’s no longer as tonally unique, though it’s still unique in other ways. That’s no reason to lose market share per se as they’re great instruments, but I think it’s perhaps a part of the story.
OK, I swear I'm not being contrary but even with pedals there is a small number of basses with uniquely inimitable tones and the stingray is one of them.
Here's my picks for those basses in no particular order.
1. Stingray
2. Warwick Thumb (go on and try to get that sound without one - I've been trying forever because my Thumb weighed 13.5 lbs and I couldn't take it anymore)
3. Rickenbacker 400x ric-o-sound blah blah blah
4. Ampeg Baby Bass
There may be more but those are the one's that I think of.
This of course is my completely opinionated opinion.