It's been several days since Debian 11 release (current stable); their release cycle started with package freeze at 2021-01-12. At this moment, it has been enough time (7 months!) to compile and put the packages into the repository for this distro, so there's no reason to not have it available.
The result for this lateness comes with a price: we are not using MongoDB for a project that starts today at my company, as the policy is to use the latest stable Debian distro (11/bullseye currently). We are not using docker for this client.
Please, take into the account that since SSPL, you are the only option to put MongoDB for those who want a trusted origin and don't want to compile their own binaries, as every major distro has kicked mongodb from their repositories for legal reasons.