Its the twitch and he also plays pretty high.Why? What about his skillset makes him an interior guy? He's 6'7 without a Calais Campbell lower body.
The only criticism I've ever heard about his edge work is that he doesn't have elite twitch. OK. He's also the longest player in the draft and plays with the fluidity of a high school DB. That allows him to cover the ground he needs to cover.
The year off allowed people to overthink Rousseau. If he played this season and stayed healthy, he'd be a no-brainer Top 5 guy like Chase Young (the only player who got more sacks than him).
You've seen many of my draft posts...I'm always a WHERE THEY WIN guy. Greg Rousseau wins on the inside. You want him to be a natural EDGE guy, but thats too much projection for me. When he lines up outside, he comes off the snap high without a lot explosion and the twitch combined with - IMO - a lack of elite bend from the edge make me lean toward what my eyes see with him on the interior. On the edge, he comes out without much of a plan of attack, lots of wasted movement, and doesn't exactly have a filled toolbox to make up for the lack explosion/twitch. Even on his sacks where he lined up outside, hes twisting inside to get space against an interior guy. You say hes got DB fluidity, but I see a lot of film where he looks really stiff.
In college, he's long, athletic, and pretty strong for a guy who hasn't fully developed physically yet. His hustle and relentlessness through trash is one of his better characteristics, IMO, and again, that all came from the interior where I thought his run defense was a strength. In the NFL, without a plan of attack and against OTs that are trained to get their hands on you, idk how well Rousseau holds up without a lot of coaching here from the outside. Too much projection and coaching to be an EDGE guy. I think the gap is closer to develop him as a long 34 end.