I think she's the only noble left that is of age and isn't a new ruling family. Edmure you could make the case for I suppose. But Robyn, Gendry, whoever is the new family in the Reach, new Prince of Dorne, and Tyrion is the presumed lord of the Westerlands now. Sansa is probably the best combo of most "well liked" and most capable of ruling out of all those people at this point. AND she'd presumably have Bran at her side, who would make her an extremely effective ruler.