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Although the modern dollar is a "fiat" currency, not backed by any other asset, like gold, its value is ultimately balcked by the fact that the U.S. government will accept it, in fact demands it, in payment for taxes
. Its purchasing power is also stabilized by the Federal Reserve, which will reduce the outstanding supply of dollars if inflation runs too high, increase that supply to prevent deflation.
Bitcoin, by contrast, has no intrinsic value at all. Combine that lack of a tether to reality with the very limited extent to which Bitcoin is used for anything, and you have an asset whose price is almost purely speculative, and hence incredibly volatile.