Why did the buddha teach the mahayana?

I know that the standard Mahayana claim about the Theravada canon is that he needed to teach it for those who were not ready to accept the Mahayana. But do all Buddhas have to teach the four noble truths, etc., as they appear in Theravada? Or will / have some Buddhas taught only (their own) enlightenment? ---------- I'm asking because I'm *confused*: whether or not it's logical to be! How can there be a Buddha without Arhants? If some or all Buddhas teach an enlightenment without Arhants, who in turn teach an enlightenment without Arhants, ***how this is any different to Arhantship***, just under a different name?