Monero's anonymity is not 100% proven nor time-tested yet and Monero is harder to liquidate for cash.  

I suspect most DNM vendors will continue to use Bitcoin, because Bitcoin has the network effect and illegal Bitcoins can be laundered over-the-counter cash through sites like localbitcoins.

http://www.coindesk.com/localbitcoins-users-criminal-charges-florida/

Cases like the above are probably the exception in terms of launderers being caught.  Whereas someone trying to unload Monero on the exchanges will probably get caught due to all the trails and investigations of coins on those exchanges.  No one in the world has heard of Monero, so you can't do cash launder deals like Bitcoin.  All these criminal vendors on the darknet have overhead and other expenses and thus non-liquid Monero does not help them to run their businesses.

As for someone like Roger Ver shilling for Monero - it's NOT the first alternate he has shilled for, he used to shill a lot for Ripple and also Applebytes, I think, and other alternates.