I’m not sure, but I guess they use a stable version in paid hosting, and by default beta is enabled for those who use open-source: it’s a kind of payment for free software.
Perhaps one commit or two can make some difference, but in the end for a larger forum (over 100k posts) a stable version is safer, even if it means you don’t have any new features that have been implemented to beta.
Not everything new = better. The stable version comes from beta, it has simply been tested. So this is a disregard for the security/durability aspect of the forum for new things.