Because we are still not sure enough whether the price will keep dropping to a certain rate or not. DCA in Bitcoin will be worthy because ow the rate has dropped enough and the probability to drop again is still high. We are not really sure how the price will be at the bottom rate. But at least, if we are doing DCA, we can utilize every drop to collect more Bitcoin.
As long as I still have money for DCA with my continuous money management, I will still do it.