and we have had this discussion multiple times. BOS does not have the $ to support the p.s. fares that JFK-LAX/SFO command. hence why p.s. is only on the NY-california markets.
Says who? BOS-SFO is one of the biggest $$$ domestic markets. And there's no shortage of premium passengers on the route. On pmUA it was one of the hardest upgrades in the system (and they ran it mostly on 24 seat 752s), even on CR1s.
B6 has publicly stated that BOS-SFO is a possibility for Mint once they've deployed fully on the JFK side.