Pass rush skill usually comes from explosiveness (quick first step), bend (flexibility to lower shoulders and turn tight corners to get around tackles) and pass rush techniques (different moves they deploy to try and fool offensive lineman).

Run defense skill is typically about strength, leverage and gap integrity (clogging up rushing lanes for running backs). Also good run defenders are excellent tacklers and have the ability to take down a running back with forward momentum without allowing him to escape or fall forward for extra yards.

Parsons is fast, explosive and has amazing bend which is why he is an elite pass rusher. I think he is strong and smart enough to be a good run defender but he just doesn’t seem that committed to it. Guys get paid for sacks and QB hits.