The main reason by far for the *private* N-reg fleet is the FAA IR. JAA have practically killed off the PPL/IR so they have only themselves to blame (and the UK authorities for signing up to it).
There is little or no maintenance advantage because N-reg owners are regarded as a nice earner for maintenance organisations. The remaining advantage is on minor mods, fitting non-STC equipment, which the CAA makes really hard and for no good reason.