Just cos they used to be measured in pounds and it's what people are used to.

If someone tells me the weight of a baby in kilos, I have no idea! LOL
Yet if you say it in pounds, I know what size it is. I guess it's from listening to my Mum talk about us as babies. It's just something that's hung on.
Plus... they still use pounds in the US and, well... it's in the movies etc.