It's a LOT of talent on one team.
The core players in this age group have been consistently in the pool and on the team for at least 5 years (some longer) - well before they were in college. Of the 5 Penn State players, 3 normally start and play the majority, if not all, NT games. Penn State winning the National Championship last year may have made it easier for the players to redshirt (as was required by US Soccer) this year.
There were another 4 regulars (starters) from Stanford, plus Taylor Racioppi (Duke) and Miki Harvey who were among the core group who would have likely started/seen significant minutes in the upcoming U20 WWC. Those 6 players opted to decline, due to the redshirt mandate. The remaining "starters" are scattered among colleges and HS - two from Notre Dame. Mallory Pugh (captain) who has stated she will compete at the U20 WWC and, in fact, deferred her enrollment to UCLA to do so, plus a few others.
Having 6 usual starters opt not to compete created opportunities for players who have been in and out of NT camps to win those spots. I believe the final "official" roster is due to be announced in early October.