I expect that going 3x weekly using an A321 instead of a smaller A320 plus brand awareness were the key reasons to the failure and axing of the Leeds/Bradford - Grenoble service. Nobody in Yorkshire knows who Monarch Airlines are yet! Hopefully this will change overtime with advertising, promotions and a good summer 2013 season! Next winter's Grenoble flights I'm sure will fair a whole lot better as Monarch are experienced enough to know what will work for them out of there new Leeds/Bradford base.
Monarch were also unfortunate as they announced the route first so clearly saw the potential it had but Jet2 were obviously looking at it and announced there plans in a rush on the same day. So the best thing for Monarch to do now, with the route been a clear failure was to retreat, re-group and return stronger...