Is that because something is wrong with the mining farms. Or miners are mining other profitable coins we do not know?
There are more profitable coins out there but you have to find them.
Yes, the thing with most miners is you need to stay on top of which coin/algorithm is the most profitable at any given moment, as most pure profit miners simply mien and dump at current market prices, this the network hash-rate will somewhat follow the general price trend of a coin.
For the true believers mining and holding this isn't as much a factor, so their hash-rate contributions will remain constant, but then they are also playing the speculation game in that the value of the coin they are mining, in this case ETH, will go up and provide more profits in the future.
While I can see how some would think this way, it still may be better to always mine the most profitable coin any given day, sell at current prices to lock in the profit. Then if you want to speculate you can use your earnings to buy the coin you wish to speculate with. You will end up with more coins in the long run.