Why am I earning less?

As the bitcoin mining difficulty goes up customers on all mining pools including our own Bitcoin.com mining pool will see a reduced revenue, you may only notice this after a large difficulty adjustment .

Bitcoin mining gets more difficult with an increasing computing power in the network. The protocol always makes sure the pre-defined trajectory of bitcoin creation is kept. The computing power is measured in hashes per second. To understand what all that means, the process of mining needs to be explained in further detail, you can read more about that here: http://bitcoin-difficulty.com/

A much more technical and in depth explanation of how bitcoin mining difficulty works on the network can be viewed on the bitcoin wiki here:
https://en.bitcoin.it/wiki/Difficulty

You can see independent data on the past mining difficulty changes here: https://bitcoinwisdom.com/bitcoin/difficulty