[pvxc]

The main reason is probably that physicists found graphene more interesting to study than buckyballs. This can be quantified by looking at the number of articles published in top physics-only journals (e.g. Physical Review Letters).

There are a couple of good reasons why so: (i) graphene has a "simple" electronic and atomic structure that has interesting features of its own (Google for the Dirac cone), (ii) graphene flakes are big compared to buckies, so it's possible to study them with common methods that physicists like -- electronic transport, crystallographic methods, you name it, and (iii) many proposed applications of graphene e.g. in electronics fall close to physics.

In short: physicist are fond of simple things, and graphene is simple. So, Nobel prize in physics.