The difference is in the scale. The US dollar is known and accepted worldwide. Countries hold it in reserve,and it's used to trade in all sorts of goods, not just the illegal ones. It's not 100% secure, but it would take a lot to shake people's faith in it.
Meanwhile Bitcoin's growth seems to be fueled by memes. Sure it's an investment, but I question whether it is a wise or moral one. Other investments pay dividends. With Bitcoin, you're gambling on either the value going up forever or being able to sell ahead of the rush.
Given how fanatical some of Bitcoin's supporters are, I predict that the crash will be followed by a wave of suicides and maybe even murder-suicides.