The most people want Bitcoin to be as a free payment and value store system.
Free and underground they are two different things - generally.
But it could be also the same in the mind of those who think in terms slavery=legal and freedom=illegal.
If you like that your money to be stolen with inflation and hyperinflation or to be confiscated because some laws which are emitted only for this and not understandable with the common logic then you can use simply fiat.
It is difficult to break the chains of slavery in your mind and to realize that basically it is nothing criminal to carry all your money with you over the border(like in a brain wallet) even if a specific law declare to be illegal.
How many millions or billions of people lost all what they saved as result of decades of work to have something for their older days or for their childrens and corrupt governments simply inflated or confiscated with a law made for this purpose.
Is it so criminal to keep safe your legal and taxed income from such government actions ?