Because the Ethereum foundation has humongous expenses it has to pay each month, and it only has ~$750k in non-ETH assets left to pay them with. After that all it has left is ~1.65 million ETH to pay those expenses with, and if it can now fund itself that could mean dumping those ETH for Bitcoins, then dumping the Bitcoins for fiat.
https://www.reddit.com/r/ethereum/comments/45bhus/so_the_ethereum_foundation_can_now_fund_itself/czwpr04[]vbuterinEthereum -- Vitalik Buterin 52 points 11 hours ago
The foundation currently has ~1.65 million ETH, plus ~$750k in non-ETH assets. 1650000 * 6.1 + 750000 = $10,815,000. Based on our current ~$200k/month burn rate, that will last us ~54 months ~= 4.5 years. That said, we are planning some substantial expansions which will increase our expenses but also get casper and other fun stuff out the door much faster, and we are also starting to get interest for corporate sponsorships coming in, which could secure us a more sustainable funding path in the long term.
And hence the snake beat itself.
The assets will come back to us. As long as Eth can't be exchanged directly for USD, it's not possible for it to take btc over.