Maybe everyone thought of Mike as just the successor to Vito,and not Don material. He came up as sort of a mascot to the Family until the Solozzo murder. His ruthlessness and attitude after that set him apart from Vito in the eyes of the others. He was
obeyed,but not respected. In the one scene where he is called Don
Corleone,it's a drunken Pentangeli,and it comes off as condescending.