Perhaps because when the X of Y naming convention was developing, the class of people at court who used it to emphasise that they owned the place in question were using French, so you get de Montforts etc. in England.

Presumably most 'English' speakers at the time travelled rarely, so introducing yourself as Snowballjane of Finchley would result in people going, "Well yes, I'm of Finchley too. We're in Finchley. What are you on about?"