Yes, politics are no doubt a factor. The coaches trust their own guys. There might be even more UNB and Alberta guys on the team if it wasn't capped at a maximum of 5 players from one school.
Don't forget they're building a team. Simpson is a shut-down centre, a big body, who is very good on faceoffs and the penalty kill. You can't say that about St-Cyr (who also has a very short fuse). Pomerleau is a very good defenceman: he's a +14 while Headrick, with many more points as you point out, is only +5. Keilly might be 6th in scoring, but only +1; UNB's Alex Goulet is tied for 7th in scoring with Stephen Johnson (who is going) and a +12, but he didn't make the team.
The coaches pick a team to have four lines of forwards, three pairs of D, and PP and PK guys. Guys have to fit roles, and can't all just be goal scorers.