Amuso remains the official boss of the Lucchese family for much the same reason that Peter Gotti remains official boss of the Gambino family. Both are going to die in prison but others in their families are content with them to have the title. Same for Carmine Persico of the Colombos, although one could argue he's had more control from prison because the Persicos are still a power in the family. But that's not to say Amuso or Gotti have no clout. Both still do.

I don't recall what was said in Gangbusters (read it long ago) but Casso didn't rat out Amuso so he could become boss. Casso was already the driving force in the family behind the throne. But it got to the point, when they were both on the run from the law, that Amuso actually put out word that Casso had been stripped of his position and authority. That no doubt was a reason for Casso flipping. And, of course, he was always out for himself.