Because Alexandria is built on the Florincoin (FLO) blockchain. This is a quote from the Alexandria.io website:

"The real innovation behind bitcoin has always been the permanent distributed ledger, the blockchain. Florincoin is a unique cryptocurrency that allows for text to be in the transaction, instead of just numbers like with Bitcoin.  Because of this, Florincoin is perfectly suited to be the index for Alexandria."

Even though the demo video shows only two types of payments (credit or bitcoin) -- everyone is "really" paying in FLO on the FLO blockchain when they make a purchase for a media asset (listen or download).

So you buy credits on Alexandria using CC or Bitcoin -- your fiat payment or bitcoin is then exchanged for FLO on the backend -- which is then used for purchases on the blockchain.

Hope that helps you to understand better.