I wonder if it's cap related. Grenier makes under the threshold for the Emergency Recall where it doesn't count against the cap whereas Shinkaruk doesn't. I don't think it's an issue right now with Prust on LTIR, but he's about to be activated so maybe if Sutter and/or Vrbata are out temporarily they'll need Grenier because they can get him up without having cap space.
If true, it really shows one of the downsides of poor cap management. You can't even afford to call up your legitimate prospects for short term injuries; you need to rely on guys who are already off ELCs and who have league minimum deals.
As a side note, the Canucks have been super sloppy with LTIR so far this year; it doesn't look like they've even bothered trying to maximize their exemptions. Only $1.4M of Prust's $2.5M cap hit is eligible for LTIR for instance, if the numbers I've read are correct.