Why aren't green eyes more prevalent?
A gene being dominant does not necessarily imply the gene is also common. 

An easy counterexample is Huntington's disease. The gene is dominant, and only one mutant allele of huntingtin would result in development of the disease. However, the allele prevalence of the mutant is low in the general population. 

In the absence of a selective advantage of green eyes over blue eyes, the two alleles of the gey gene should remain in genetic equilibrium, and therefore the number of people with green eyes will not increase. In fact, a similar misunderstanding applies to the disappearance of blondes/redheads. In both cases, the allele frequencies will remain unchanged in the absence of selection pressure.

The initial imbalance in the allele frequency could be due to many reasons. Population bottlenecks, genetic drift, and historical selection pressure could all be valid answers.