I just figured that from a race organisation point of view calling something a trail race might attract more competitors than calling it a fell race, especially from the non-fell running fraternity. I think fell races do fix an image in peoples minds, one that could put mamby pamby types off maybe
I know the route of the Settle Loop 'tough trail' race really http://www.settleloop.org.uk/ and the first half especially is fell running pure and simple and the whole route is largely at a 400 metres plus height, making for grim going if the weathers bad. Its a fell race really but they just call it something different to be different and attract runners