Is BTC.

They have plenty altcoin listed on their casino, most of the time why everyone still using BTC because on some country their local exchange only offering both of cryptocurrency (BTC & ETH) + all about the market order book. If they a big whale, BTC or ETH have the order book more than altcoin that why even the fee a bit expensive they still using it or they got forced to use BTC/ETH for the transaction because the local exchange only offering that.