"But how effective can they be if they fail to reach 15% of a population" was in reference to the fact that almost 15% of the DC population doesn't have a bank

"But how effective can they be if they fail to reach 15% of a population" was in reference to the fact that almost 15% of the DC population doesn't have a bank account which the author took to mean they can't use UberX or any other Uber service. This isn't strictly true but the barrier to entry without a bank account is high enough to dissuade most of this population from using Uber services.