Obviously because of the complications involve with BTC. Not everyone around the world want to remain anonymous in their ventures and realize that banking are scam. They feel banks and markets are perfect and swift. So they use banks for quicker remittances. Moreover even out of the 6.2% I think only 20% might only be using bitcoin for transactions everyday rest of them might only be using it for store of Value. I think problem lies with the transaction times and costs. Maybe Lightning Network could provide a solution for this problem and payments could become swift and convenient.