There's trust and then there's trust.
Michael trusted Tom to protect Michael's family (which is also Tom's family) and he trusted that Tom wasn't privy to whatever evil plans had been hatched such that Michael and Kay were almost killed. Although he gets some sort of sick pleasure from castigating Tom he never doubts Tom's loyalty.

He did not trust Tom enough to allow Tom to take any aggressive actions independent of Michael to identify or neutralize internal threats in the Family. Not only was it a question of control (Michael wanted to do that himself) but probably one of competence. Tom is almost as smart as Michael but not quite and nowhere near as cold-blooded.

Michael doesn't know who in the Family has been compromised. If he had told Tom everything Tom may have let something slip that could have tipped Roth off. Or Tom devises his own plan which possibly alienates a loyal member and lets a guilty one escape.

I still think that someone besides Fredo (maybe Rocco?) or one of Fredo's men had to be involved in the attempt on Michael's life. For example not only do we have the whole drapes thing, but we also have the swift and quiet killing of two hitmen, the fact that they were on the estate in the first place, the fact that they avoided detection for at least some hours after the Communion was concluded and the fact that they had knowledge of which home was Michael's, all of which would argue that unless Fredo was some sort of Crouching Moron, Hidden Badass there was more than one traitor in the Family.

All of this would have been going through Michael's mind so he would have avoided giving anyone more information than they needed about his next moves or future plans. And as Michael correctly and cynically surmised, Tom would be content to be considered a "real brother" and sit in the Big Chair for a while without asking any other questions.