It's mostly about endorsement deals - look at a copy of
Gear magazine, do you think any of those people they feature
really use Vintage as their first-choice guitars?
Or look at people like Dave Mustaine, Paul Stanley, or Dan Donegan with their ever-changing guitar endorsement deals. Mustaine basically just takes the same guitar from Jackson to ESP or Dean or wherever he is now, then says it's much better than the cr@ppy previous version. Jerry Donahue has done the same going from Fender to Peavey to Fret-King, Van Halen from Music Man to Peavey to Fender/EVH.
To be fair to Steve Vai, Joe Satriani and Paul Gilbert, as Nolly mentioned they've all consistently used DiMarzio for many years, and worked with Steve Blucher to design models to their own requirements - so they don't
need to go anywhere else.
On BKPs specifically, Chris Broderick was the classic example - he was using BKPs then suddenly switched to DiMarzio when he landed the Megadeth gig. I'm pretty sure
that was an endorsement deal. Jon Schaffer from Iced Earth was using BKP, then switched to WCR - maybe he liked them better, maybe BKP wouldn't make him a signature model(?).
Misha Mansoor from Periphery is a fairly well known BKP user - he's contributed to this site and mentions BKPs in magazine interviews. He does also use DiMarzios (and maybe Seymour Duncans too?) in other guitars though. There probably aren't many players who exclusively use one brand.
