Because this ‘currency’ is volatile like a small cap stock. This is the thing I always get a runaround about from Bitcoin advocates. Some say it’s a great currency, some say it’s a great investment. Well, by definition if it’s good at one thing, it’s bad a the other. Investments should provide good long term appreciation, and currency should be stable ideally not fluctuating in the double digit percentages in a given year. Bitcoin, you’re a fun experiment, but what’s the point of your existence?

Bitcoin hopes to be a currency, but isn't one.

People who have bought e.g. a pizza with bitcoin are examples of what NOT to do with Bitcoin. The very fact that Bitcoin holders expect it to rise in value makes it unsuitable as a medium of exchange.

I guess for the same reason you implicitly compare BTC to USD or EUR, it needs to be compared to something to give it a value as it does not have an underlying one