Why exortionists are still using Bitcoin for this stuff instead of using Monero is beyond me. Maybe they don't know better?
I think that it has more to do with the fact that they know their victims would know more about bitcoin, be able to easily set up an account and buy it with fiat money. With Monero directly, that's not as easily accessible... hardly anyone has heard of it. I'm 100% sure that you will never see Monero listed for buy/sell on regulated brokers like Coinbase.
Also, the reverse is true with extortionists more easily being able to sell bitcoin for fiat somewhere. Or just convert it to monero themselves.