The problem with STN is that it's a boring airport.
I haven't had to spend that long going through security since the 90's and when you get to the otherside there's nothing to amuse you.
Gatwick is quite a 'fun' airport in that there's always plenty to keep you occupied until boarding. STN is just cold and unfriendly. It is missing a certain something.
We went through STN last year because it was £100 cheaper than LGW. We'll happily fork out that £100 to have a better airport experience at LGW in future.