I believe content of his books is now less accessible to modern kids. Internet (Youtube) doesn't promote his works, ideas. Flexagons are rediscovered, instead of being included into school curriculum, etc.
I think "less accessible" is a disingenuous way to put it. The internet has made his works more accessible, not less. The problem (for the Gardner estate, I suppose) is not availability, but discovery - there is significantly more math content to digest (for free!) and therefore lower probability any one budding mathematician is digesting Gardner content.