Because bitcoin is not easily traded on margin.
Bitfinex apparently has margin trading now.
its not good enough, i tried i could NEVER take a position because their was no one leading, the min someone was lending it would get taken.
go lead your coins or money and get a good % back
These days the number of lenders seems higher. Whenever I look at the current lending market, there's someone with a few thousand dollars available at the "flexible rate".
That said, I'm definitely taking advantage of the current interest rates
