The WSL2 architecture video lists two motivators from WSL1 based on number of Github issues. One of them being file performance and the other syscall compatibility with some specific examples being the ptrace syscall and Docker compatibilty. https://youtu.be/lwhMThePdIo?t=755

The HN submission for the youtube video also has discussion about the same topics. https://news.ycombinator.com/item?id=19868282