Because Bitcoin is the tool for normal people to stand up against hard-core capitalism and capriciousness of the central banks world wide.There is an elite that is fearing the potential of Bitcoin for the masses.
why bitcoin is still illegal in many countries of world

Is there any solid reason behind this ??/
No, it's simply a transfer of money without third party

The idea that it can take down central banks is a joke, banks and government are intertwined.