Gravity perhaps?
Too many bitcoins in your wallet and it becomes too heavy to move?
C'mmon people, try to keep up.
Fact A. Mining difficulty follows price, at a certain delay needed to provision hardware, etc.
Fact B. Cost of mining is directly proportional to difficulty
Fact C. The majority of miners are not long in Bitcoins and seek short term guaranteed profit
Put A, B and C together and you should see a stable high price for more than a few months will force miners to sell most of their inventory to stay in business.
Regarding C, even if you cover mining expenses out of your own pocket, that's still a capital inflow that enters Bitcoin. You just bypass the market, that's all. The miners simply can't do that long term for any arbitrary price. Say you cover the price of rig today, and it's wonderfully profitable. 6 months from now the cost of running your rig is the same but the share of hashing power and corresponding btc revenue has dropped to the point where you can just break even. But the rest of hashing power growth is in the hands of miners who aren't long and will hit the exchange every day.