In the late 1970s or even the 80s the New York families weren't that much in decline yet and still very powerful. Their real decline started in the 90s. However, for the Bonannos the 90s were a good time and it wasn't until their entire administration flipped that everything fell apart. Montagna's status meant enough for people to align with him. However Desjardins himself didn't have much to do with the Bonannos at that point and neither did his guys which is why they didn't care for him to be the leader, and when Montagna tried to take out Desjardins it was simply kill or be killed.
"It was between the brothers Kay -- I had nothing to do with it."