You know what they say about "in matters of taste.."
Sanderson is huge. People know about him and buy his books in droves. I can't even figure out which Hamilton you are referring to.
I get a few search results for that name and fantasy, but no one that seems to stand out. The top search result I got was a fantasy football news site. Compare that to a search for Sanderson and fantasy, where every hit on the first page was the Sanderson in question. If I add the word author, the top result is an urban fantasy vampire romance author, which I'm guessing isn't who you're thinking of, and is a genre the TV studios seems to not be into these days. A bit further down is a Duncan M. Hamilton, who looks like a more likely fit, but he doesn't seem to be talked about much. I searched for his name and there was basically no chatter about him. No thread on preferred reading order or anything else that usually pops up. Again, compare that to Sanderson's presence on social media. People talk about his stuff pretty much everywhere.
If you were a TV exec and was looking for something to adapt, which of those two options to you figure you'd pick? The one people not even into the genre has often heard the name of, or the one even someone interested in the genre can't quite identify when trying? Hamilton might be the greatest fantasy author of this generation, but name recognition is a pretty big deal in these decisions. Currently, it seems like a stretch to imagine a TV exec would even know Hamilton exists, whereas Sanderson is extremely well known and has been actively working his way into that side of the industry for many years now.
You might well be right and we're all missing out, but from a TV exec's perspective, Sanderson is obviously the right choice at the moment.