ZipPy, the initial implementation of Python on Truffle, was an intern project developed by Wei Zhang [1] while doing his PhD at UC Irvine, and continued by other students for a while. It was later re-implemented from scratch by an Oracle Labs team. I think that the main reason for a clean re-implementation was that the Truffle framework had evolved enough where it was easier to start from scratch than to rebase. Also, ZipPy was reusing the Jython parser, whereas GraalPython is using ANTLR [2].
[1] http://thezhangwei.com
[2] https://medium.com/graalvm/how-to-contribute-to-graalpython-...