That is a good question, and, to be honest, I cannot give a totally clear answer because I don't know all the rules. I can, though, give some information.
I think one rule might be (not sure) that your agency home address not be a vacant lot. I don't know how that rule would work, but I would guess that would be an issue.
When UTD became an agency out of nowhere, one thing it had to do was become insured. That requires an investigation from the potential insurer to determine if you are an acceptable insurance risk. I know there was an issue with that because some of their standards were different from existing agency policies. One specific example was their failure to teach CESA. They eventually overcame that problem and gained insurance. Thus, their agency is insured, and their instructors can be insured individually.
Once they became an agency, other agencies immediately recognized their certifications. I have certified technical divers who crossed over from UTD, and PADI spelled out to me exactly what I could credit them for based upon their UTD certification level (and why). Other agencies do not recognize SDA's certifications. I do not know the details as to why that would be, but they don't.
Their website says that their online academic program is recognized by all other agencies, and students can take their certificates of completion to any other agency and have it accepted. That is not true. No agency accepts their online work, and the PADI website says that specifically.
Their website used to have a list of dive shops around the nation that they claimed had specifically agreed to accept their certifications. Several were in my home state of Colorado. I called each one of them and asked about it. Not one of them had ever heard of SDA. Not one of them had any idea they were listed in this partnership. Not one of them said they would accept anything from them. I assume there were complaints made after that, because SDA changed the wording on that page, and instead of listing them as partner dive shops, they said that these were high quality dive shops that they recommended so that students could complete their training with them. It was the same list, but this time it did not overtly claim they had any kind of formal agreement among them.
So, no, I don't know exactly why they are not recognized as a legitimate agency; I just know that they aren't. I know that as soon as other agencies came into existence, PADI and other agencies acknowledged their legitimacy, but that has not happened in this case. If this sounds like the kind of operation with which you want to do business, go for it.