The NBA, like any major sportsball league game is a tangled web of licences, sponsors, player deals, and very silly people at the top of them all who don't want people to have fun.
For example, let's say there's a player named Michael Porridge who appeared on the 1991 team of the Floridian Guinea Pigs.
Let's say that while he did good at curling, the rest of the team sucked and the team was dissolved by the next year. He doesn't want to be associated with those dark times, and refuses to licence his likeness.
That's one possibility. Now imagine an entire league of has been players.