Bitcoin is not an asset, Bitcoin is money and the definition of money is a currency (ie. a medium of exchange) that can be used as store of value. This means because Bitcoin is acting as a store of value and also because it is in its early stages of adoption, its value keeps growing against fiat making it a suitable investment.
Calling Bitcoin an asset is like ignoring everything else that it offers which are far more useful.
Bitcoin has not been granted the access to be called money, Bitcoin is a digital asset in which you can invest to yield positive in return, secondly Bitcoin is not acceptable by every country, its a few countries that embrace Bitcoin currently, so on that note, it's not generally accepted as the way traditional currency is generally accepted as a currency...for now Bitcoin is asset, because the purpose of some countries purchasing bitcoin is for economic reserved, so, from my perspective what's been reserved for future betterment is called asset, so therefore Bitcoin is an asset.
Bitcoin is a currency and was created to serve as a currency. It is the volatility of bitcoin and it's limited supply that made people realise that they can gain more HODLing bitcoin than using it to buy coffee. This is where the game changed and people now start using bitcoin more as an investment asset than a currency.