Because I don't think he's as good of a defensive player as he's made out to be, we can fill the fourth line either more cheaply or by shifting someone like Helm or Gator down as the team (hopefully) gets better, we're already seeing Larkin get more PK time than last year, and we need to find someway of getting $5m of value out of Nielsen (whose pretty good at the PK to boot, so it's not like we're shoe horning him in there).
Also, while Sheahan had an awful year last year, I still like him more and would rather see us give him another year to turn it around. If he doesn't, I doubt his trade value is going to get any worse so it's not a big deal on that front.
It's not that I see Glendening as someone we more desperately need to lose, it's that he's the guy I'm most okay with losing who gets us what we need (cap space). I agree with Shelby, though, that Sheahan is more likely to be moved, though.