Caused by the UK CAA and also believe JAR's. These require new aircraft to be certified with wet runway performance figures. As that is/was not required in the USA it would cost huge amounts of money and performance penelties if it was not properly done. So somehow they agreed on a simplyfied performance reduction by adding a weight penalty AND reducing screen height in order not to suffer to much on take-off perfomrance.
There was a year ago also a discussion about, but can't find it