At Stansted the airbridges were provided on Piers 1 & 2 by the mid 1990's before the LCCs had really started, so it wasn't known then what we know now. The newest Pier 3 (opened 2 years ago) has no airbridges and was designed with LCC's in mind. RYR share Pier 2 with others but are the sole Pier 3 operator.
Apart from the extended turnaround time airbridges are expensive to provide and maintain and also need additional resource to operate them so it is not in every airport's or airline's business model to have them. They also can take up valuable ramp space that may be better used for other puproses.