I always perceived that it was split along class lines (this is the UK, after all).
The true working-class tabloids were the Sun, Mirror, and Star.
The Mail and the Express seemed to be more for people who were doing a little better, were often (correspondingly?) a bit older, but were rooted in right-wing/small island mentality, and who would probably have been uncomfortable with the image/implications of moving to a broadsheet.
The Daily Express used to be a broadsheet, and was always thought middle market. It went tabloid after Beaverbrook died - in the 70s I think. Somehow, it had a reputation for decent, honest but right of centre reporting. Quite how I'm not sure as Beaverbrook had infamously told a 1948 enquiry into the press that he ran the paper "purely for the purpose of making propaganda and with no other motive". He may have been Murdoch's role model.
Ever since it went tabloid it's been downhill all the way, and right down the toilet after Desmond.
The true working-class tabloids were the Sun, Mirror, and Star.
The Mail and the Express seemed to be more for people who were doing a little better, were often (correspondingly?) a bit older, but were rooted in right-wing/small island mentality, and who would probably have been uncomfortable with the image/implications of moving to a broadsheet.