When it's a legal requirement to never made profit off of a property again, they won't risk it. You're basically saying it doesn't make money anyway, which is why they're (potentially) burying it so they don't lose any more money by paying residuals etc. But Star Trek is a business, intended to make money. And putting a character in a show that's been written off with a promise not to make money with ever again? That's a legal disaster waiting to happen.
Off topic (to an extent) but to me an interesting tangent - how far would these sorts of restrictions go? Presumably anything that was developed Pre-Prodigy but then used on the show would be excepted from it? Or would Janeway (whose most recent appearance is as a semi-main on the show) be deemed to be profiting from Prodigy making her relevant again?
Or is it simply the case that it is only new and original ideas//characters developed for the show which cannot be used again?