Jens Ritter used to have a blurb somewhere on his site in the early 00s about slanting making the strings hit the magnetic field in a way that affects the eq response of the pickup vs that same pickup laid in perpendicular to the strings.
I don’t know if he still stands by that hypothesis/claim. He was seeming to be rather particular about exact angles for each pickup, IIRC, more slant on the neck pickup than the bridge one.
It also can possibly function to widen the aperature of the mag field exposed to and affected by any given string, possibly leading to a fattening similar to the difference between Tele pickups & P90s? Maybe that’s what Jens was driving at?
Position on the speaking length of the string is the strongest reason for a player to want pickups slanted one way or another.
As I started years ago EQing my D & G and just enjoying whatever Thunder results on the lower strings, I could imagine some sick-of-thick-studio-tone punks of the 70s-80s really wanting as much middy trebly gank on the G & D but still get some thunder on the low strings?
As bholder said, regarding why people with money to invest in the profitable musical instruments market in the pre-Bush/Quale years trying to get that sweet <30 year old male demographic money: the reason would’ve been 100% “novel aesthetics”, aka it just looked cool. Those products got made, marketed, distributed, and sold for a bit over a decade before flooding Music Go Rounds & pawn shops in the 90s-00s.
Warwick Thumb, I’m 98% sure, was designed in the 80s… in one of the most 80s-o-riffic countries on the planet. Even if its designer was a jaded marketer and only slanted the pickups to sell basses, I’d really like to try a copy with bladed 6 string pickups with an absurd amount of slant: both pickups near the neck at the G, both close to the bridge at the B. Probably a PITA to mix live, but fun on the couch.