Why didn't anyone believe Leo about Marcus.

Right from the beginning Marcus was trying to hangout with them, inviting himself to their house and asking them to come over to his house. If I am trying to keep something a secret I am suspicious of this person and even if he does seem okay I don't take any chances and keep him at a distance. Marcus even destroyed his guitar and blamed Leo and when he got caught came up with some lame excuse that the Davenports bought hook, line and sinker.