Another reason is that Michael is more deliberate than Sonny. He would have wanted to know more. IOW I know you betrayed me but why, who else was involved, how long has this been going on, etc...the same sort of conversation that Tiger and Elin or Jenny and Mark Sandford have been having.. shhh

And since Michael had already lost the logically most rational guy (his bodyguard) to do the job on Fredo, would he have done the job himself?

"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.