in my opinion, the reason why bitcoin is not a legit currency is because the common currency is being regulated by any institutions or government. And this is very different with Bitcoin that is not regulated by any institutions or government. Beside that, bitcoin also very risky and very susceptible with hacker and scammers.
Bitcoin cant be a legit currency because it is decentralized, it means that it cannot be controlled or manipulated by someone, and also bitcoin is volatile, means that its price is changing rapidly and unpredictable if its going up or down. Government are the one who regulated something to make it legal, if government cant control it, then it cant be legit.