There's no person or company that is paying out profits to investors.
This. In a Ponzi scheme, you pay money to someone, and they pay you a return. But that return that they pay you is actually stolen from someone else.
Bitcoin is just like a stock or a commodity. You buy a piece of a limited resource, or you're given a piece of that resource, and then you re-sell your piece later. If you pay $1000 for a Bitcoin and you're only able to re-sell it for $100, that might make it a bubble, but it doesn't make it a pyramid.
this. Another point: Transparency. In a Ponzi scheme investors do not really know, how the profit is made. The scammer tell people something and the investors have to trust the scammer. In Bitcoin: Open Source + properties of Bitcoin, which makes it valuable (fast, cheap, anonymous, etc.). Maybe some investors don't know about this and just invest, because they see the huge price raise, but this is their fault. The informations are there and easy accessible by everyone, in a ponzi scheme never.