As everyone touched on Solozzo thought he couldnt' be touched and Michael being a civilian and McCluskey being a cop would almost guarantee a safe return.

As for the Bocchicchios Don Corleone had the connections to employ them in returning michael to America, on the other hand however Solozzo was set up by the Tattaglias (but really by Barzini) as the point man and the guy to take the fall first if everything went wrong. Solozzo was also an associate of the Tattaglias who weren't as well connected with the Bocchicchios as DC was and they wouldnt' extend this type of protection in any case if the Tataglias were connected like this with the Bocchicchios. That is because this would probibly cause a war between the families.