As far as I remember the original plan was to only maintain both in a transitional phase, with the aim of fully replacing the Java edition. A simple plan: bring Bedrock to feature parity with Java, add a modding API that satisfies 95% of use cases, then force everyone onto Bedrock. The feature parity is mostly there, but modding in Bedrock seems to have become a non-goal, and Bedrock has so many bugs that if your platform offers the choice between both Java is still preferred even if you don't care about modding.