I have a question...I have asked once if there is a death penalty in New York, if there is...why most of the Mafia Bosses did not get the death penalty?
Why Joseph Massino did?(That would be the reason he turned to be an informant)

Maybe because the crimes they were convicted of weren't serious enough. Loan-sharking, theft, gambling set-ups and hi-jackings are IMO not the crimes to make someone deserve the death penalty.

Of course they killed and gave orders to kill, but I don't think they got convicted many times of those murders because of the buffas.

Enzo's right: either they haven't been convicted of a capital offense, or the government couldn't find witnesses that linked them directly to murders. Massino escaped the death penalty because he cooperated with the government. Gotti was convicted of complicity in the murder of Big Paul Castellano, but it was a federal charge, and the feds have very little precedent for executing defendants in federal crimes. Plus, it would have been difficult for them to get a jury to convict him in a death penalty case on the basis of Gravano's testimony, IMO.
The only major organized crime boss to get the death penalty was Louis (Lepke) Buchalter, who went to the chair in New York in 1944.

That's why Lepke got the death penalty.
One time he didn't have a buffa.