It is supply and demand. There is very little friction in being able to buy bitcoin via circle so users will naturally want to buy on circle more then they wish to buy via say bitstamp where you need to bear the cost of wiring USD to them, paying a deposit fee, and waiting however many days to have your account credited.
With circle, if you have a debit card attached to your account, then you can buy BTC instantly.
Makes sense. I'm used to seeing a bid ask spread. I guess with circle pay there's just a market order and you're given the market price before you transfer money in. I wonder how they'll end up generating their revenue.
