It's pretty simple.
EZY operate out of Gatwick, which has limited slots. Yes, EZY are big there but if the NCL flight wasn't performing how they wanted then I'm sure there are plenty of other routes they could use the slots for which would be more profitable for them.
It's a little disappointing they didn't give it atleast another 6 months but that's the problem with a constrained airport such as LGW, airlines put their planes where they can get the best return because they HAVE to - it makes financial sense.
In regards to STN, well if they couldn't make LGW work then I don't see how STN or LTN could work. If they thought they could then they'd have introduced one of those instead of LGW