May be criminal activities using bitcoins can not be detected due to decentralisation and a law and order of the countries can be affected.
Since every technology has pros and cons like that way different countries may face different challenges to make it legal so can go against it.
I think many governments are afraid because bitcoin csn cause a lot of changes and defer from the norm. At present, cryptocurrency market poses a threat to the taxed income of some countries because crypto is something that is quite sensitive to regulate due to its charscteristics. That is why many countries nowadays are gunning to regulate or tax crypto use for their advantage.