DeCicco was a soldier at the time when Bilotti was made underboss. A promotion like that would have raised eyebrows among the capos close to Castellano. He looked for loyalty first and foremost in the new underboss. So he was definitely looking to dismantle the "Manhattan faction".
Gotti struck because he had to. He found himself in a vulnerable position. It was literally kill or be killed at that stage. And he had to seize power as well. There was no way he was willing to face repercussions from Castellano loyalists (not to mention Gigante) as a mere captain or even worse, as a mere soldier.