Triablogue gives reasons “Why Markan Authorship Wouldn’t Be Fabricated.” One is the person of Mark himself:
So, Mark was a relatively minor figure who was remembered for some disreputable behavior and/or an embarrassing physical deformity.
Regarding Peter as the source of the gospel:
It fails to explain why the gospel was associated with Peter to begin with. It also doesn’t explain why the gospel wasn’t attributed to Peter himself or a more prominent and/or less problematic disciple of Peter.