By "cerebrum" I assume you really mean the cerebral neocortex, since the cerebrum includes other subcortical structures that themselves are quite distinct (e.g., the basal ganglia).
Simply, the neocortex and cerebellum have completely different functions. The cerebellum performs a particular class of computations that are very different from the variety of computations performed by the cerebral cortex. They have completely different laminar structures, with neocortex being a ~6-layer structure, the cerebellum has just 2-3. There are a much higher diversity of connections and cell types in neocortex compared to cerebellum.
It seems like an optimal configuration for the cerebellum is a very high cell density. More cells doesn't necessarily mean "better", because the more cells you have the less space you have for connections. There is no unused or empty space in the neocortex: if you packed in some more cells you'd have to lose something else.
Doya, K. (1999). What are the computations of the cerebellum, the basal ganglia and the cerebral cortex?. Neural networks, 12(7-8), 961-974.