Never saw Rogue One but from what I understand we know this guy bites it, right? Was he a super compelling character to justify wanting to see his pre-corpse shenanigans?
He is painted as an uncompromising spy of the Rebellion movement (ie, the means justify the ends) which is something not so commonly represented in the media. Unfortunately, the movie does not spend enough time on most character's backgrounds, hence the reticence to get a full show based on him.
For the record, I love the actor (Diego Luna) and liked his character. He once did quickly mentioned that some of his lines were altered last minute, and that he trained for weeks/months certain "accent" style for Cassian, just for the 1st day of filming, to being told to switch to a completely different one.