Lot of miners have bought their equipment using btc, so they want to earn btc not usd. Also, lot of miners don't sell everything they make, at this price, you have to be quite desperate to sell the coins.
Miners want to make a profit. If a miner bought their equipment for 1000 BTC in January, mined 1200 BTC with it, and paid 100'000 dollars of electricity, he invested 900'000$ and got back less than 600'000$, so he lost money. Even if he loves BTC, he would be better off today if he had kept their 900'000$ in the bank; he could buy more than 1800 BTC with it now.
Miners decide whether to keep, buy, or sell BTC just like any investor or trader. If there are people selling for 460$ now, why wouldn't they sell too?
EDIT: they --> he