Because bitcoin's value isn't derived from the fact that it is legal tender.

It is sort of like gold. It's a universally accepted currency that people use, because it can't be gamed by central bankers or devalued simply by printing more of it off. That property of gold is translated onto bitcoin as the capped supply which is enforced by code.

Also, its utility as a decentralized currency online makes it valuable. This doesn't mean that the prices of BTC is always at par with what is fair at that particular time, though, as we saw with last year's price bubble. But bitcoin does have value, albeit not from a legal sense.