Because of the same reason why rates droppend when the price rised
But also because more coins are wagered when the price is low, thus they have a bigger budget to spend.
I would like to know from online casino owners that does the rise and fall of bitcoin really effect that much in wagered coins, I know it will effect but only if the rise or fall is of over 300$ .. because its normal with bitcoin to bounce in between 200$window..! but I would like to hear from owners how much does it really effect because if the wagered amount really go up then the rates should be modified.