Why is the income of Dogecoin getting lower and lower?

For Dogecoin merge-mining rewards with Litecoin, F2Pool currently adopts the PPLNS model to calculate Dogecoin rewards separately. There is no direct relationship between Dogecoin and Litecoin's mining revenue and there is no fixed reward ratio.

Rewards mainly depend on the difficulty of Doge mining and the actual output of the mining pool. As the difficulty of Dogecoin continuously rises, it may lead to the decline of Dogecoin rewards.