Why did the darknet markets disappear?

i remember that every now and then i would read about a new site that is growing, and another that was going down.

never bought anything, nor was i particularly interested, other than the stories (like silk road).

and now i have noticed that they have started to decline or disappear altogether, i think a couple of them literally said "we are retiring" and closed the sites.

is there a particular reason? too many scams?