LCY is one of 2 "London" international airports with a London postcode (the other being Heathrow). People accept it as a high cost entry point into London with limited opening hours that are designed to meet the business traveller's needs.
SEN is a regional airport that seeks to be accepted as an alternative to the other 3 out of London "London" airports (Gatwick, Luton and Stansted). All of the other 3 are open 24/7 and have facilities to cater for passengers when there are no flights (24 hour opening for food and drink outlets).
A foreigner turning up to a "London" airport would reasonably expect 24 hour opening. When the word spreads that SEN is not 24hr then the perception will spread that SEN is a regional and not a London airport.
Much better to be viewed as a regional with good links to London, than a London Airport with restricted terminal opening times. As another poster pointed out, the cheap fare is more than outweighed by the fact that there is nowhere to stay warm and buy a coffee and a sandwich overnight. Who wants to book into a hotel for 4 hours? I suggest nobody when Luton, Stansted and Gatwick all cater for the overnight traveller.