Why DC and marvel characters so popular in movies ?

Established IPs, it's marketing 101. People are more likely to express interest in something they've heard of before, regardless of their personal level of familiarity.

I'm curious as to what they do differently, movie to movie. I'm not into them so I wouldn't know

They have on colorful costumes and have simple personalities.

because the characters are stupid and people are stupid hence they can relate