By definition, to execute a 51% attack on Bitcoin, you would need to buy computing power greater than 100% of the entire network's current computing power. In other words, you would single-handedly double the global demand for computing power in this market.

Is there enough supply readily available to satisfy a doubling of global demand? How much would it cost to bring such computing power online? How quickly could it be done? Wouldn't the price of computing power skyrocket?

EDIT: Meekro's comment elsewhere on this page makes essentially the same point in a more concrete manner: https://news.ycombinator.com/item?id=22161500 -- I think his comment is better; read it. Also, see bencxr's analogy with trying to control 51% of global oil supply: https://news.ycombinator.com/item?id=22161575