NiceHash sometimes has good prices and sometimes they don't, especially for algorithms that run best on cards like the 1080ti.  As you explained, NiceHash is a marketplace. The earnings that miners receive are dependent on the prices that buyers are willing to pay for that hashing power. Recently, buyers have favored the Dagger/Ethash algorithm that runs better on cards like the 1070/1070ti than the 1080/1080ti, making NiceHash less profitable for miners with the 1080s.

Also, NiceHash doesn't pass the profits it makes from selling hash rate to buyers to the miners - that is Nicehash's fee for providing the service and how it makes its money. Since buyers want to make as much profit as possible from their purchased hash rate, they often bid as low as possible for that hash power.  So there will be times when miners can find more profitable coins to mine on other pools that will also convert their earnings to BTC.  While my personal daily earnings have fallen with the rest of the market, I still earn at least a dollar more per day on each of my 1080tis than what Nicehash is currently offering.