Why did Mahayana leave out the Pali Canon?

I'm quite familiar with Theravada and Zen but I can't understand the historical reasons for not keeping the original Buddha's teaching as a core. The schools do not contradict each other, so why reinvent a wheel? And also the Pali Canon feels more explicit. For example Heart Sutra of Avalokiteśvara is beautiful but it will not make as much sense to a newcomer as reading a Nikaya or two.