Probably because most British people can't get inside one if you paid them
And if you make them wear them, they will never fly with you again.
Technically, a drysuit is a very good solution but it's strictly for the most dedicated anoraks.
Even lifejackets are a pain. I have the best GA ones, with the reasonably breathable fabric (the £100 ones from SEMS rather than the £50 ones) yet I still find nobody will wear them except for the actual water crossing. And I don't blame them.
Also I am not sure about how visible you are bobbing up and down in one of them - unless you have a 121.50 beacon tied to yourself with a piece of string, or some other means of detection.