I originally figured they didn't make it because they had a losing record in the ACC regular season. No team from the ACC with a losing record made it. But then I noticed that two teams from the SEC (Vandy and Ole Miss) made it despite having losing records in conference. This despite the fact the ACC was rated by most to be the top conference in the country. So I guess maybe it does have something to do with the fact the ACC has no representation on the selection committee.
I think having no representation on the committee is a big part of it. The B1G also had no representation on the committee, and Rutgers didn't make the tournament with a 44-15 record and a 42 D1Baseball RPI. Not a great RPI, but for a north east program? Also, Maryland seemed underseeded at #15 with a 45-12 record and #9 RPI. Usually I would think that's a no brainer top 8 seed for a B1G team.
I think they also look at aggregate conference record, so including the tournament. In that regard, NC State was at 17-16-1, ahead of Georgia Tech, Florida State, and Wake Forest. I think the goal posts can move year to year, which makes it difficult for programs to set schedules that will benefit them come selection time.
An NC State friend that I work with threw out the possibility that if NC State was in the tournament, they'd constantly be talking about how the CWS ended for NC State last year, and the NCAA probably has no desire to continually re-hash that decision. Far fetched, perhaps. But the NCAA has made self-serving decisions many times before, so I wouldn't entirely discount that theory.