Why did the buddha teach the mahayana?
Why did the Buddha teach the mahayana? No, He did not teach Mahayana. Nor did He teach Theravada, Sarvastivada, Pudgalavada, etc. and the 18 or so schools that splitted from His original Teaching/Dhamma after His passing away. Fortunately for us, all the different schools still share the same "core" regarding the Discourses and Discipline: 3 characteristics, 4NT, 5 Precepts, 8NP, 12DO, etc. Of course there're certain doctrinal disagreements among the schools which is expected and unavoidable but if one sticks to those pillars of teaching, one'd find out that they have a lot more in common than difference.