Simple: because the vintage germ is stubborn. For example, this 1973 is for sale in Germany and I can't stop thinking about it.
You like blue things, don't you ?
I've never played - let alone owned - a JMJ Mustang but went through several Mustangs during the '80s, and in all fairness it's one of two Fender basses that I wouldn't have a problem buying sight unseen, the other one being the '68-71 Tele Bass.
Why ? I've never played a bad Mustang, something that I couldn't say about its more famous/expensive cousins.
My $0.02 only...
Happy shopping.