The columnist, after wondering why the hell this topic came up after their LAN party, admitted, if I recall correctly, to also going bottom up, and she asked around the office and found that something like 90 went top down.
I was totally surprised by this question. It never occurred to me that anyone would use the opposite direction from the one I use. The thing is, I have no idea why I go the way I do...presumably I was taught that when I was young enough to need assistance, so presumably I learned to use the same direction as my teachers, but that's before my earliest memories.
If the columnist's observation, and the original questioner's observation, and what I and a friend observed when we asked around at work, hold up, then it looks like there is a heavy skew toward top down.
So...how does bottom up persist? Unless there is some factor that tends to make bottom up people tend to marry other bottom up people, most bottom up people will marry to down people, and so most children of bottom people will be raised in a mixed direction household. So shouldn't half of them take after their top down parent, and so shouldn't the bottom up fraction be decreasing?
Or...maybe some people who themselves use top down find bottom up works better when they are teaching a young child, and that leads some children of top down households to end up bottom up?
There's probably some fascinating psychology research possibilities in this, but because we generally don't wipe in public or talk about it, it gets lost in the cracks.