if one start converting a lot of Steem in Bitcoins right now he would cause the Steem to lose value in the short term.
If this project is going to have success (becoming a competitor of Facebook, Reddit, Twitter), those people who sold out their steem power would have just made a bad business, while the ones who bought it would have made a nice deal :)
This happens thanks to the steem/steemdollars/steempower system.
We also have to keep in mind that it's our interaction that is producing value. We produce this value everyday but google/facebook/amazon etc literally steal us this value. And they have to do this because of the economy based on debt, that forces the economic actors to maximize their value in the short term, because there is always a debt to repay somewhere.
Ok. But now you gave another explanation. And this one does make sense ;-).