Cracks appearing in a tyre is an indication that the rubber is degrading and should be changed as soon as possible, as a cracked tyre could fail under load. It is possible that your exceptionally low annual mileage is contributing to the problem. When a car is left parked for extended periods of time the tyres can flatten off on the section that is in contact with the road, and particularly if a car is parked outside the tyres are exposed to UV light and temperature cycles that cause the rubber to degrade. When the car is driven the tyre is rotating, is at a more constant temperature and is constantly flexing, all of which help to retain its flexibility. Although the front tyres have not suffered in the same way it is possible that they will eventually do so - they may have lasted longer because the front tyres in a front-wheel-drive car do more of the work.