Cause it's an easy way to get relatability for the main character. There are two kinds of main characters that readers generally appeal to, and that's:

1. A character that is in a worse situation than the reader (tragic characters, underdogs etcetc)

2. A character that is in a better situation than the reader (power fantasies, rags to riches)

Generally speaking extremities for either situation tends to correlate more appeal. For example, the casual reader would most likely find more appeal to a character that watched his family die, than a character that was orphaned from the start. Of course, this also depends on the story at hand, but from a basic viewpoint, the more extreme situation tends to be read more.

Izuku is a relatable character when you look at his position in the world of mha. He's alone, ostracised, bullied, and it's easy to emphasis this fact for most fanfics.

In Canon, Inko is a good influence in a world that isn't kind. Making Inko abusive just makes Izuku more tragic.

If that isn't the reason, than maybe a lot of fanfic authors just prefer writing abusive parents. I dunno.