The difference is how compliance is handled for cloud providers. Complying with the AGPL requires releasing the source code for the program you're hosting as a service. Complying with the SSPL requires releasing the source code for "all programs that you use to make the Program or modified version available as a service, including, without limitation, management software, user interfaces, application program interfaces, automation software, monitoring software, backup software, storage software and hosting software" under the terms of the SSPL. This is effectively impossible for any cloud provider to comply with (it would require a massive engineering effort to rewrite every single piece of software used to convey the service), which is why the OSI didn't approve it as an open source license.