Because pressure works on girls?
The rapes?
The rapes?
This might just be saying something like "bullies don't excel in school", given what we already know about socioeconomic demographic trends WRT fertility. With the wrong study design they could even get this result if bullies have lower fertility than some SES sub-group they also tend to be a part of. One would suppose they adjusted for that, but sometimes papers make it out with some really weird oversights.
These two things aren't as related as people think they are. One can be a "nice guy" and still not be a pushover. Being assertive is not the same as being aggressive.
less manly <--------> more manly
pushovers most men bullies
Rape is using force, threat of force, and/or intimidation to get sex - aka rape is bullying for sex.
Why would we discount that fundamental connection for some sort of imagined "pleasantness" - particularly when the entire premise relies on there being a single woman cloned billions of times rather than assuming that women (who are also human) have a wide range of preferences and personality traits (like the men you discuss)?
As much as I appreciate that everyone has their different tastes. Being mounted by a heaving silver back, with deep grunting and howls, sweaty and lumbering, isn't everyone's cup of tea.