I thought I had two answers for you for a moment, but I've just thought about them more and I've just argued against them (*COUGHschizoCOUGH*).
I was going to say that firstly, it's easier for lorry [did you know they don't use the word "lorry" in America?] drivers to see because their cabins are high up. But regular height lights would still be fine (only the low ones you find in France would be out of sight, but they're part of a set of two anyway). Then I thought that maybe it's so that the queue [another word they don't use] of traffic behind them can see what the lights are doing. But then I thought 'Why does it matter to people far back in the queue what's going on at the front?'.
So in short, Spanner81, this is a completely useless post and I have absolutely no idea. Maybe they're just nosey... hehe.
Hang on, I've just had a thought. On fast roads, maybe it's so that you can see the lights changing a long way in the distance so you don't have to brake suddenly?