The FBOs all have contracts with (primarily ? 90% 90+% ) a small group (3-4) of “Distributors.”
In general, the FBO - Distributor contracts forbid the FBOs from selling any fuel that does not come from their contracted Distributor.
If the Distributors refuse to bring their FBOs G100UL - - the FBOs are unable to stock and sell G100UL avgas.
So, that very small group of Distributors who have a classic (choosing words carefully) “dominant” market position - - have decided not to bring G100UL Avgas to any of their FBOs.