This interesting passage was taken from the booked "Iced" By Stephen Schneider. Here they give another reason why Massino wanted Sciascia dead.

"Sciascia fell out of favor with Massino because he increasingly sided with the Montreal crew against the wishes of its New York masters.The tipping point appeared to be when Sciascia supported Rizzuto's refusal to send men from Canada to kill the New York Post employee,which forced Massino to delegate the job to Salvatore Vitale.
After Scascia's murder, Vitale travelled to Montreal on orders of Massino to try and placate Rizzuto,who was angered over the Sciascia hit.
Rizzuto was offered the formal position of captain of the Montreal crew. In a stunning display of defiance Vito turned the offer down. By the end of the 1990's, Rizzuto was no longer sending tribute money to New York. This was more than symbolic. While the Bonanno family was in steep decline during the 1980's and 90's,due to deadly internecine battles,the Montreal mafia under Rizzuto was growing in strength, wealth and international reach. Rizzuto's relationship with the Bonanno family was now irrelevant to modern business structure that Vito had been building.It was a bit like a waning alliance on the postal system without the advent of the internet. The sixth family had not only eclipsed the wealth and strength of the Bonanno family in New York,they now seem to have extricated themselves from the hierarchy and were truly setting themselves apart from any of the five families of New York.But as Rizzuto was abandoning a relationship that Montreal has had for more than fifty years,his former bosses in New York were now dragging him down with them."