Pete Hiltner and Nick Bylin are also missing from the roster. I believe Hiltner's departure was due to a lingering back injury. I seem to recall that Bylin was on the roster for spring ball, and has enrolled at UND for the fall semester. He must have had personal reasons for leaving the team.
As for Simmons, he had good size and came highly touted, but he may not have been meeting the expectations some had for him. He may have recognised that and decided to move on. Sometimes great success in High School stems from the system the player played in and/or the natural gifts a player has. When stepping up to the next level, tougher competition, under a different system, those natural gifts are not enough to be successful.
I'll just throw this out there as well. Would the departure of Greg Lotusz been a factor in the loss of these O linemen? I know from talking to some of them that he was probably the most popular and respected coaches on the team.