>The broader context is it sounds like even after the huge and unexpected financial success of the film they didn't go back and reward the actors beyond their initial stipends.

This is common for small budget films that suddenly make it big. The actors sign a contract that grants them a lump sum instead of residuals. Sometimes the contract even gives them a choice: do you want more in your salary or do you want residuals? If the actors don't believe the film will make a lot of money, salary is the better choice.

I don't know the specifics of BWP contracts but industry wide that's pretty common.