Simple logic.

Usage does not determine the function of something. For example, there are fruits which can be used in making wines and  other stuffs. Likewise with Bitcoin, it can be used as a currency which is its main purpose because it is a digital currency in the first place. But it can be also used as means of investment due to its volatile market value. It is just that the usage of Bitcoin as a mode of payment is still limited to some countries and to most of the places, Bitcoin being used as an investment through blockchain technology is what is more convenient. This gave it an impression being more of an investment than digital currency.