because it is limited , unlike fiat currency
The very best answer I think.
Other reasons include the speculative Ponzi games in which the most of the supporters believe and are trying to involve new believers, mostly.
That is why many investors are loving the way on how bitcoin moves on. And basically that is why many are keep on investing into it because for every price movement of bitcoin that is making them generate profit very fast. For those newbies it is not new that most of us became a victim of ponzi's since we are lack of knowledge on how things goes on in bitcoin.