It’s not a stock though. Just because you can buy or trade it doesn’t mean it has a market cap.
What’s the market cap of gravel? It might be $1,000 a ton but you can’t multiply that by the amount of gravel in the world and get a meaningful number.
OK so how about gold? How much of that is there? Are you counting ore, how about gold a mile underground? Same problem.
The point is that market cap only applies when you’re talking about a security that represents a claim on a fraction of a whole asset that exists independently of the security itself.
The concept of a market cap is meaningful when adding up all of the stock yields 100% of a defined thing.
In that case you can surmise that the market cap represents some kind of representation of the value of that thing. The lower limit on market cap is generally considered to have a relationship to what that asset is worth.
But Bitcoin doesn’t have intrinsic value. It’s not a thing at all. If you owned all of it you literally would have absolutely nothing at all. Someone else could just start using the same code and call it bitcoin. There’s no asset there.
Hence, no market cap.