This is a result of the SSPL license that MongoDB has used for the core server since 2018.
According to the text of the license, specifically section 13, any company that provides MongoDB database "as a service" (which is what you are asking about) is legally required to provide all of the other software involved in running and operating the service under SSPL.
This is an impossible requirement to comply with in most cases since some of the software that a company may be using to provide the service isn't developed by the company providing the service, and is either proprietary or made available under a license other than SSPL. Therefore the company providing the service wouldn't be able to provide that software they use under SSPL.
For example, it is my non-lawyer understanding that someone providing MongoDB-as-a-service using a completely GPL stack, and publishing all of the pieces, would be in violation of SSPL because the pieces would be provided under GPL and not SSPL.
A company could comply with SSPL by developing their own system management software and releasing all of it under SSPL. But there's a weak business case for doing something like this.
MongoDB itself, being the copyright holder of its software, is not bound by SSPL's requirements and does not provide the software it uses to provide MongoDB-as-a-service under SSPL (i.e. MongoDB Atlas is proprietary).
Not even version 4.0 which was released in Aug 2018 can be found.
MongoDB 4.0.3 is the last version licensed under AGPL. This version could be provided as a service without the provider being under SSPL obligations, but users would most likely demand the current 4.0 release (4.0.19 as of this writing) which is under SSPL, plus a provider offering 4.0.3 would be on their own as far as patching any security vulnerabilities found in MongoDB since 2018.
