"what is it that prevents the East Midlands being able to hold on to services other than those offered by leisure and low cost carriers?"
Bottom line is that East Midlands has a more rural and leisure oriented catchment than say BHX or MAN.
Both BHX and MAN were late to "wake up and smell the coffee" with regard to the low cost evolution of short-haul and this worked to the benefit of airports like EMA and LPL for a while. This situation has now changed. Had this not been the case at BHX then EMA today would be more like CWL (+ cargo).