Because it's still new. And actually a solid percentage of bitcoin users still don't 'trust' bitcoin.
its new and there is no central bank to control it. Im afraid if crypto really become our world wide curremcy it would make economy really bad. Just think about how goverment would helping poor people , goverment would not be able to adjust the price anymore, maybe they can by making rules for it but still the money is world wide i believe no one could interven it anymore and goverment power will be reduced.