I suppose it depends on what your definition of express is, what you're used to and so on. I think most people in that part of the country equate express to 125mph trains, that are either direct or 'more or less' direct.. which places like Milton Keynes and Peterborough do have.

I think the reasons are mix of the two above you mention. No ecml which for what can only be described as a decent economic hub is a missed opportunity. Train station a bit of a pain to get to which extends real life journey times, although they are clearly trying to improve that and Cambridge North isn't bad.