An advantage of WASI is that it mostly works on non-WASI runtimes too, as it can be compiled to one or more normal modules (with the multi memory feature). So

An advantage of WASI is that it mostly works on non-WASI runtimes too, as it can be compiled to one or more normal modules (with the multi memory feature). So a possible reason is that maybe they are not interested in offering general interfaces but just a single python specific one. In this case they could have likely used wasi for that too but I can see how it could have been seen as an overhead