In the US, Disney's divestiture of these types of assets, enable them to hold out for a "reasonable commercial value". No clue if there are similar laws in Mexico, but to your point this offer is not "reasonably commercial". Because selling the assets to Televisa, creates the same condition in reverse (Televisa would then have the dominant position), another buyer has to surface, and pay a reasonable amount for the Fox Assets. TV Azteca might be that company, but so far, no deal, and Disney was able to easily get a 6 month extension from Mexico. It feels like dragging their feet is the best option for Disney, and maybe this all goes away (they no longer have to divest).
According to Yahoo Finance, TV Azteca is not profitable. A quick look at their balance sheet shows a degrading position. Shareholder equity has dropped from 8.5 Billion at the end of 2015, to 3.5 Billion at the end of 2018. Year to date 2019 hasn't been any better. The company is deeply in debt. So, financing and taking on the Fox Assets may be a challenge, which TV Azteca may not be able to do on its own.
Something to think about.... Sinclair probably took a look, when they were looking at the RSN's. Would Comcast be interested? Where does the buyer come from?