Box set potential

[FuzzyPlum]

It was probably nothing more than Billy just never being needed for the tribute concert.
Billy is not a big enough name to merit an involvement in the event. Rick was only a part of it due to his reverence of Peter. It also sounds as if it was a peace offering from Mick to Rick as they'd fallen out a couple of years previously (or at least grown apart).
All other guitarists involved were A-listers or iconic players. Billy wouldn't have added anything.
I'm sure it would have had nothing to do with Stevie Nicks' opinion.