The cult of Richard Wagner is the primary reason for this. Whatever the composer might have envisioned for the future of his house, his family has spent the 130 years since his death carefully cultivating the religion. Cosima froze the Festival's production style in the 1880s for decades. The Festival can't even perform Wagner's first three operas there, because Wagner himself forbade it.
Well, Wagner's been gone a long time. Perhaps the moment has come, or will come some day, to rethink the repertory performed. Would continued family control and such limited repertory be tolerated for any other composer?
I do realize that there might be a practical limitation on how much the theater can be used. It's 140 years old, it's wood, and I have no idea how fragile it is or how much use it can tolerate. But let's run the Bayreuth repertory thought experiment anyway.
1. Put Die Feen, Das Liebesverbot, and Rienzi into the rotating repertory. There, we've had a nearly 30% increase in what can be performed! More importantly, these operas shouldn't be ignored, however much Wagner might have wanted to disown them. He wrote them and they are part of where he came from. We can understand his mature operas better if we hear them more often. Are they really worse than any number of other operas that hold the operatic stage for one reason for another? Faust? Elisir?
2. Perform early German Romantic operas, especially Weber's Oberon and Freischütz. Schubert wrote a few operas, reputed to be weak. There's Schumann's Genoveva. How about Marschner's Der Vampyr? There've got to be others worth hearing.
3. Perform the works of the French Wagnerians. Wouldn't you love to hear Reyer's Sigurd, and works by Chabrier, D'Indy, and others?
4. Perform works by Wagner's German and Austrian successors, especially those who are underperformed. We don't really need more Strauss performances, but James Conlon might have a few ideas. Recovered Voices at Bayreuth!