My understanding is that there are two issues:

1. Moving the drills from place to place has to be done with a helicopter after the ice breaks up. It's more expensive than sliding the rig over the ice.
2. Drill pad placement is more restricted after the ice breaks up: you need dry(ish) land

There may be more issues but I think that these are the major ones. As Hoov pointed out in his excellent PDAC report, there are other great options for spending the remaining $13M of the $25M flow-through funded exploration budget for this year than just drilling. He specifically mentioned airborn surveys as being valuable for finding new and re-evaluating existing drilling targets, using updated technologies. Noront sure does have enough claims to map across their vast holdings in the RoF.