It was taking a chance to wait but the Family was weak at that time that Michael returned. It took a while to openly (and secretly) rebuild things , get money coming in again and get Michael plugged into all of his father's political power. Vito and Michael also wanted to ensure that their enemies underestimated Mike as a not too bright kid just content to take over the business.

The Don's death let everyone drop their masks and worked for the storyline but if I recall corectly there's a part in the book where Michael and Vito are discussing things and Michael says everything's ready and that he found Fabrizzio. The Don asks him about the other thing (referring to Carlo) and Michael responds he's ready for that too and that even if the Don stepped in to prevent it that Michael would do things on his own.

"When the snows fall and the white winds blow, the lone wolf dies but the pack survives."
Winter is Coming

Now this is the Law of the Jungle—as old and as true as the sky; And the wolf that shall keep it may prosper, but the wolf that shall break it must die.
As the creeper that girdles the tree-trunk, the Law runneth forward and back; For the strength of the Pack is the Wolf, and the strength of the Wolf is the Pack.