It's because Hermione is often portrayed as a strong character. So when she cries, it shows a different side of her. It adds depth to her character and makes her more relatable to readers.
Nezuko is initially 12 years old in the manga. As the story unfolds, her age doesn't change significantly in a traditional sense, but her character development is quite remarkable.
It's popular because their relationship is complex. Sakura has had a long - standing connection with Naruto. Their friendship has had its ups and downs, and fans like to explore the emotional depth of Sakura's feelings for Naruto through her crying. It adds drama.
A simple Nezuko and Zenitsu fanfiction could be that they go on a journey together. Along the way, they encounter other demon slayers who are initially skeptical of Nezuko. But Zenitsu stands up for her, explaining that she's different. This shows the depth of their friendship and how Zenitsu has come to trust Nezuko completely.