It is one of the first cryptocurrencies.
Its ability has been tested over time and has proven to be worthy. Moreover, people who use it find Bitcoin trustworthy, the most important part of being a cryptocurrency is that everyone needs to believe in you and actually use you. Bitcoin has always been used and has mostly never had trouble. As long as there are people who are using it for their purposes, no matter what it is, Bitcoin (and other currency with the same reason as well) will still be used by people all over the globe.