Stargate simply isn't that big. Its fan base has always been modest compared to other sci-fi/fantasy franchises, meaning that the expected audience for a new show isn't that big, especially since what fans seem to want is a continuation of the previous franchise that respects all that backstory and lore and might not be welcoming to newcomers as a result. Combine that small audience with the expense of making sci-fi TV that looks good, and the business case for a new Stargate show isn't as strong as some fans seem to think.

MGM had years to bring back the TV franchise after Universe was cancelled, but they didn't. They have The Handmaid's Tale and are involved with Vikings, so it's not as if they're not interested in genre TV.