That’s because we still use Bitcoin is the main trading pair, and most people track their prices through bitcoin trading pairs (Yes your Blockfolio showing your USD amount is tracked using bitcoin trading pairs).
We just need fiat to coin trading pairs for everything on a large exchange, and we can finally break direct ties to bitcoin.
Good think Bittrex is adding USD deposits and pairs, and Robinhood is opening feeless crypto trading using USD pairs.
A step in the right direction