This may be because they are a compromise. The overall level of crash protection offered by convertibles appears to be a compromise, perhaps because they have to operate in two totally different modes. When their poorer crash test and user performances are considered, convertibles can be poor value for money. If a car has one of each restraint type, the older child can occupy the forward-facing child seat and the younger child the infant restraint. Both then have the protection of a restraint system specifically designed for the purpose, rather than a compromise for one child.
Convertibles are very large, especially when they are being used as rearward-facing restraints.
Fitting a convertible into a small car can mean that the front passenger seat is virtually unusable for an average size person. Useful for Convertibles can be useful for occasional child carers, such as grandparents and taxis or rental cars when there is a need to keep a restraint on hand in the car for those occasional trips with children, and when you have to be able to cope with a wider range of ages in a single restraint.