There was a thread on this subject a few years ago, and the consensus seems to be that if you're not able to stand for the duration of your journey then you shouldn't be travelling by tube as pregnant women aren't will or weak and don't need to be patronised by being offered the chance to sit down.
I think that some people make the assumption that the above is the case and most women would be insulted to be offered a seat. Others won't bother to offer seats because they don't 'see' other commuters, or just don't care.
However, not offering seats in London isn't a new thing.
Fifty years ago I lived in London and travelled to work by bus everyday, generally with the same people, and, because of where I got on, I usually got a seat.
For a few months, every morning, the same pregnant woman would get on the bus a couple of stops on from me, by which time it was standing room only.
I was always the only person who offered her a seat. If I didn't have a seat to offer her she would have to stand.