Fundamentally, it's the same reason that there's no C, D, R or I class availability on your flight either: revenue management. BA thinks the flight is likely, on average, to be more profitable if it does not accept any further requests for cheap tickets but takes its chances that someone will buy an expensive one because they have to. This is a bit of an oversimplification, of course - but that's the fundamental underlying philosophy. It often means that the higher cabin will go out with empty seats even though there are people who would have been happy to upgrade into them at a cheap price (but not an expensive one).