Because:

1. decentralization of exchanges (and at times of other currencies where decentralization matters, such as STEEM) being not even a consideration.
2. wallet freezes at all exchanges
3. higher fees than in fiat
4. scammers, pumpers and pushers
5. inflation rates that exceed fiat at times, or conversely, expensive or slow transactions
6. the occasional 51% attack
7. Digital currencies are fiat money too, and they usually lack even the paper or coins that simplify life
8. Much harder to use than sovereign currencies and harder even than using the internet to manage a bank account and trading.

And there are more reasons that I forgot to mention or still need to know add more here, to why the entire digital currency market is rather overvalued.