Microsoft is throttling Windows 10, version 20H2 availability to provide all users who want to upgrade with a positive experience while downloading and upgrading the OS. The October 2020 Update was released on October 20, and it immediately started rolling out to users who manually check for updates via Windows Update (seekers) on devices running Windows 10 1903 or later. The ongoing throttling process is part of an effort that will end once Microsoft is convinced that the rest of the users left to upgrade aren’t affected by remaining compatibility holds.
Source: https://www.bleepingcomputer.com/news/microsoft/microsoft-is-throttling-windows-10-20h2-availability-for-all-users/