Because if it is not - it is simply a centralized database of payments. Like an SQL table of transactions. This has been a solved problem for decades. Visa and MC processing servers all use cryptography in many parts of the stack, and yet no one calls Visa a cryptocurrency because of that. This is why if you see anyone use the word "cryptocurrency" when in reality they talk about a legacy centralized (thus lacking all decentralizations perks) database - you know they are assuming you have no actual knowledge about the area and are trying to buzzword you.