Most if not all packages in the package manager show that the only version available is the one that is installed.
An example is that the HDRP 10.1 is available for 2020.2.0b8+ and the editor warns that there is a newer version, but the package manager only shows v8.1
Burst and Timeline are the only two that seem to have other versions available.
There is still work going on on that end. Please excuse the state of this for the moment. Manually adding the packages to the manifest is the recommended workaround for now.
I discussed this a bit on the Unity Discord already but I imagine there are two things going on here right now:
Unity wants to move more packages to Unity core, basically ship only select version for that engine version by default and if you want to change it, you have to change the manifest manually (either via text editor or use “add git package” -route from PM and just put the name of the package there). More about packages as part of Unity core here: What is next for us at Unity with Scriptable Render Pipelines .
afaik this also isn’t new to 2021.1 but also can be seen on latest 2020.2 betas already as it ships with more packages included by default.