I reckon a mixture of/or the following;

- Anastasia power within the family - as head of murder inc, a strong faction of the family, his brother big on the sea front etc and his reputation as a killer

- mangano due to the above didn’t have the balls

- was boss for decades which made him sluggish to move (like Castellano was)

- obviously waning influence in the family to make a move. Both him and his brother got it so must’ve been served up to Anastasia by their supposed loyalists..

In that life if you allow any slant by an underling without retribution the writing is on the wall. Mangano allowed him to argue without whacking him. Other see this as a sign of power and go where the wind is blowing. Like gotti refusing to hand over the tapes and talking loudly about paul behind his back. This must’ve got back to Paul but he was too worried about his cases etc not thinking that he could get it as easily as his predecessors got it. He probably thought it’s 1985 no commission sitting boss is going to get it in this day and age. Mangano probably thought the same until he got it.