Considering the lack of effective medicines and the fact that Prince George (later Duke of Kent) was close to his brother John and may have witnessed a seizure or two, George and Mary might have felt that the calm atmosphere away in Norfolk would help John. The advice from doctors may have proposed a regular daytime routine, a loved nurse to look after him, toys and animals, no excitement etc. That wouldn't have been possible at Court as John got older and of course he wouldn't have been able to go away to school as his brothers did.