I would imagine that somebody somewhere has done some research and determined that x% of households now watch on widescreen televisions (or at least TV's that are usually switched into widescreen mode), where "x" is a large number that's fairly close to 100.
Can you still buy 4:3 TVs? Rear projection models?
I think in the UK for a long time everything was kept 4:3 safe, and for the last few years of analogue transmissions, everything had thin black borders top and bottom (14:9 ratio). I think for a while some of the SD versions of channels kept their graphics in slightly different places to the HD versions - it wasn't a straight down-conversion. But in recent years, if you have a cut-out 4:3 image, then you're missing all the score boxes.
And to be honest it makes sense. Having the box floating "in the middle" of the picture is odd if the vast majority of viewers are seeing a 16:9 image.
Adam
PS The cheapest HD sets in the UK are way cheaper than CRT sets ever were. I remember going out and helping mum pick out our first colour set sometime around 1985 (yes - really!), and it cost much more than a supermarket cheapie is today.