sent a question to nicehash about the drop in earnings and got the following reply from them:
Hi
You are selling hash power and the market for your hash power is defined by what people are willing to pay for it and not what bitcoin itself is worth.
For example, there is a lot more miners that there was 3 months ago.
That means, there is more miners that want "a peace of crypto's pie" and so you get smaller "pieces".
On the other side you have buyers that are not willing to pay higher BTC prices for hashing power.
Please keep in mind that NiceHash is a marketplace that connects buyers and sellers of hashing power - we do not set prices.