I suspect WestJet is the only real disappointment to STN here - if they weren't chasing them then their business development people need shooting as they made no secret of the fact they were planning a London service. Still it's a competitive airport market now and that's what happens. The boys from the North West continue to find out the hard way what its like to be a small(ish) fish in a big pond.
Saying that I am not surprised WestJet chose Gatwick (much lower risk - maybe actual, certainly percieved), however they still have to secure/buy the slots at Gatwick - but as a new entrant they will get some priority there.