Overwatch is by far the smallest category to choose from for the daily quest. Only 9 heroes (counting with Mei), compared to 17 for Starcraft, 18 for Diablo, and 30 for Warcraft.
Additionally, the 3 Nexus heroes will never appear for a daily quest.
Suggestion:
Make the daily quest Overwatch OR Nexus heroes
this will at least bump the small number up to 12, offering a little more variety to choose from.