Because it is expensive to those of us who enjoy the extremely green sport of sailplane flying. Also it is an illustrative example of the inane legislation we have suferd from over the past ten years.
It is a legal requirement to insure all aircraft over 500kg in weight against hijack and terrorism; this includes single seat gliders! Although the risk of hijack of a single seat glider must approximate to zero, the average premium is about £175 per year on top of the usual insurance for airframe and third party. This is nonsense. I understand the legislation was supposed to be aircraft over 5000 kg ( five THOUSAND ) not 500kg. but the error will not be rectified.