Tesla apparently wanted to include a version of FSD beta into the mainstream release so that owners could subscribe to FSD and experience FSD beta. At the time they made that decision, FSD beta version 11.3.6 was the software that had most recently "gone wide", so they included that.
When Tesla thinks that it's worth the effort (and assuming 11.4 is considered better than 11.3.6), they'll take 11.4 and make that the version that goes with the mainstream build - and they'll be working on 11.5 or 11.6 by then, but the development version won't have "gone wide".
Why Tesla wanted to make FSD beta available to subscribers is something that only Tesla can explain. Perhaps it is to drum up more purchases. Perhaps it is to placate regulators about something. Perhaps they're testing integration of FSD beta with the mainstream development path. Perhaps something else entirely.