"Why would you think its obvious?"
Because it has worked that way with every single past version of Trisquel ever: You can upgrade from one version to the version immediately after that one. Skipping versions is not supported so if you were on an old Trisquel you'd need to first upgrade through all of the intermediate versions first before finally landing on the newest one. (Or if someone is on an old Trisquel it might be faster to reinstall rather than upgrading through lots of different versions.)
