Because Roon for Windows, Mac and Roon Server for Linux version 2.64 is the latest production release.
Version 2.65 is an early access (beta) release.
You can opt to use the early access builds if you want to - but you have to migrate the Roon Server and all of the Roon Clients and ARC installs that you use to the early access build as well and, being a beta release, issues are not unknown.