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Why not? It's fast, and it's free... at some point it could be.

Because the Steem blockchain can only handle 1000 transaction/second.
(source : https://steem.io/documentation/performance-and-scalability/)

If you compare this to Visa which can scale up to more then 50000 transaction/second.
(source: http://www.altcointoday.com/bitcoin-ethereum-vs-visa-paypal-transactions-per-second/)

Visa is here for quite a while. I'm sure in less than half that time Steem can do wonders.