Because they know that doing so would spike the difficulty and dramatically lower the expected value of bitcoins mined per unit. They can sell them for $20k a

Because they know that doing so would spike the difficulty and dramatically lower the expected value of bitcoins mined per unit. They can sell them for $20k a pop now, claiming that you can make that $20k back in a month of mining - but if they sell out and all their customers start mining back their $20k immediately, it'll take far longer. In the meantime, Bitfury already collected their money and are laughing on the way to the bank.