Maybe it's because they don't consider you to be in the correct genre, straight handlebars has maybe got you marked down by them as a non serious cyclist not worthy of acknowledgement. Put a set of drops on and you might get more waves. Just my theory.
There is also an element of north/south divide.
When I rode with my brother and sister-in-law along a tow path in Oxford I was one of the few people who spoke to passers-by, usually to warn them of my approaching presence from behind.
Sister-in-law commented 'no one speaks around here', and I can see why to an extent because there was lots of people.
Oop north there are far fewer folks about, so acknowledging those you do meet is less of a chore.
One might draw a parallel with public transport, far more chance of falling into conversation with someone on a train in the north than on, say, the tube in London where speaking to a fellow passenger marks you as a weirdo.