Because Bitcoin is inherently deflationary and there is incentive to not spend it, that pushes its purchasing power into socially/legally marginal areas of the economy. This is why it's the transactional unit of choice for online illegal drug purchases, prostitution, extortion/ransom demands, etc.

All of this of course, makes Bitcoin even more socially marginal. But yes, this is an area where Bitcoin excels for purchases.

(and to clarify, this is not to say that I personally think that drug purchases should be illegal although I have no problem with extortion demands being illegal, just pointing out the place of such transactions in the social milieu as it exists)