There are people using Bitcoin as currency, so it didn't fail. But it's not a rule and each person can use Bitcoin as it better fits their needs. Maybe it looks strange that people use a currency as investment option because we see our traditional currencies losing price each new day, being unprofitable to hold, otherwise we would have fiat as a glimpsed investment too and not only to pay for our needs.

More people will start using BTC as currency if it's interesting for them. It can be achieved when more stores from different sectors start accepting BTC as payment and consequently when the demand for the digital currency increases.