Also, why is SSPL not open source? As far as I can tell, it mostly just seems like the APGL, but taken to 11. I can definitely appreciate why it's a problematic license, but I don't think "not open source" is its problem.
edit Here's an informative StackExchange comment. [0] Apparently it's a good deal stricter than the AGPL, and introduces much more legal uncertainty.
Elastic obviously does not publish their own management infrastructure code under SSLP, so the reason for this license to exist is to make the playing field uneven as opposed to all the other free software licenses.
Basically, they can benefit from your code on top of and around ES, while you can't from theirs. This is actually the result of dual licensing but with other free licenses, at least the symmetry is maintained for the core product.
I am not making a judgement call here, just explaining what the distinction is.