Have you seen the video or read his explanation why he (in high school at the time) reacted the way he did in order to protect his sister? Plus, he has done a complete 180° and been an outstanding athlete and teammate in college. If you watch the video, it wasn't as bad as people are making it out to be.
I did see the video, and the woman was helpless on the ground and no threat to his sister as he repeatedly punched her. Sure, he could have done even worse, but it was pretty darn bad as it was.
Again, maybe he is a changed man, but it is not just explainable as a simple youthful indiscretion, and it is a red flag a team has to at least be aware of. Certainly an NFL team that signs him has to be aware that the NFL, who governs the teams and players, has such a dim view of the guy they refused to let him participate in the combine.
But again, even aside from all that, the further a team gets into the draft the lower the odds of finding immediate contributors, and given that the team doesn’t have a first round pick, the Cowboys need someone in the 2nd round who can contribute right away.