Because bitcoin is not totally accepted in the whole world. Although bitcoin is pretty convenient as payment online, not all places in the Earth accept bitcoin so the only way is to convert bitcoin into dollars or other fiat currency. Conversion could stop if it becomes legally accepted everywhere.
Bitcoin is not accepted in some countries because nobody has set about Bitcoin. Bitcoin prices are also very volatile, this is what causes Bitcoin denied here and there. Whereas we know all Bitcoin is very profitable. There have been so many successful people since knowing Bitcoin. Bitcoin deserves to be preserved.
