Belichick is a former ST coach, and focuses on ST more than just about any other HC in the league. He often keeps multiple ST only players.
If you're a guy he drafted as a core ST type and you don't make it, chances are you're not as good as fans think you are based on a few preseason plays.
Not to take anything away from Grugier-Hill, who could still contribute if he makes it to the PS and then the 53 eventually. He made some plays for sure. But the fact that he made a few ST plays and still got cut says a lot about both the team's evaluation of Grugier-Hill's ability to be a gameday active player and the other guys who are deemed core ST players. It probably boils down to guys like Grissom, J.Jones, King, E.Roberts (whichever of them make it) also being likely ST only guys on gameday but offering either higher caliber ST play or more ability to be defensive backups if/when needed.