As a savings vehicle, depreciating currency (i.e. bitcoin) is an ideal place to park assets. Why in the world would I, or any other sane person spend the coins, knowing that tomorrow, they will have greater value.
The second factor is that bitcoin appears to be a giffen good. As its price continues to escalate, more people will want to own it.
Bitcoin isn't really a Giffen good because it has utility in that it is very good as a medium of exchange, and as the value increases it means larger amounts can be sent / recieved and converted to fiat without creating wild volatility on the exchanges.
If a really desirable swanky house is a giffin good, is it still a giffen good if it has a nuclear bunker and underground farm built underneath it?