This is simply because of the supply limits on exchanges. People are unable to send their Bitcoin Cash to exchanges in order to sell them on, resulting in increasing prices. Due to the way market capitalisation works, this does not mean that there are USD 10 billion in Bitcoin Cash (although you haven't explicitly said that). Instead, it is simply the price of the most recently sold BCH (or most recently sold aggregate thereof) multiplied by the supply - which is not quite the same thing. I wrote a Steemit post about this distinction recently.