Thing is that the prospect of standing downstairs on a packed bus when there are seats upstairs can make the decision for you. Well, that and the buggy owners/tradesmen with tool boxes/people with trollies/big backpacks for school/etc are already occupying the spaces marked for disabled/elderly/less likely to be able to stand seats.
You can't come down the stop before because then there's the bulk of people leaving that stop expecting you to get out of the bus with them - can't really stand in front of the doors.
It may just be London buses that are like this, but going to work is not an easy prospect where you can leisurely place yourself on a disabled seat downstairs and then expect the driver to wait whilst you are the only person to stroll off the bus at your own convenience. It's a cross your fingers that the ten to seven hasn't been dropped, isn't full to overflowing with schoolkids because the twenty to seven was dropped or the driver just sails past because he's not in the mood, then get on and head upstairs unless you want to stand in direct contact with other people, their backpacks, tools, buggies and other stuff and hope that you don't get sent flying, get shoved off the bus by the people trying to get from the back of it to the exit doors and, hopefully, you don't do what I did last week and nearly faint due to a combination of lack of air and somebody slamming their backpack into my buggered shoulder.