Because governments could not take full control of it unlike with cash. Cash is a hundred % controlled by the government. They can produce as much cash as needed. Bitcoin on the other hand has a fixed supply and cannot be owned physically by any person or entity. Which is why Bitcoin is not totally legit in most parts of the world.
fiat has a guarantor in the form of a gold asset and is recognized by the feds and many other official institutions, while bitcoin has no guarantee whatsoever for fluctuations, the government can not be liable in this regard and it feels dangerous to the state's finances if it becomes a legitimate means of payment.
there needs to be strong consolidation from all countries to regulate bitcoin because until now they realize the potential of bitcoin is very large, but moves very wildly without being able to produce policies from official institutions.