About two-thirds of owners revert to a normally driven car when replacing their hybrid and I think there are a couple of very good reasons why this happens and why its probably more prevalent in Europe than the States.
Firstly, hybrids can't be boost charged from the mains. So the only time that you're running on electric is when enough power has been harvested from the regenerative systems to build a charge in the electrical system. Some of the main engine power is also used to charge the batteries. The net result is a car that isn't as good to drive as a comparable petrol car and one that also rarely runs all electric. Being able to plug a hybrid in and charge its batteries from the mains would mean that many shorter journeys could be undertaken on electric power only.
The second problem is that hybrids don't generally live up to their claims of fuel economy gains - as a result buyers are finding that the cost proposition doesn't work and abandoning them in droves. In Europe we have wide selection of family sized diesel cars that give excellent returns on mileage, which is why I think hybrid churn is probably worse here than in the States, where that option isn't available.
The answers are easy - add a mains charging circuit and build hybrids around small, super economical diesels.
Otherwise hard won hybrid customers will continue to be all too easily lost...