The same reason for which people generally FUD Bitcoin or r/btc FUDs SegWit. They are stupid and/or ignorant to 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.
But.... There are regulated exchanges that do have Monero, ie: KRAKEN.
Also the extortionist have a support line for explaining the "client" how to do it, there's not that much overhead for explaining how to obtain the Moneros instead of Bitcoin.. plus... when we are talking about this sums and corporates, I am sure they have access to cryptocurrencies "experts" that can obtain it for them and even provide OTC in a matter of hours. In fact I am sure it is not their financial director who personally do it... they hire someone for that (usually the same security firm that does the incident response).
Also, the get BTC now and convert to Monero later is not good. Look what happens with Wannacry ransom Bitcoins with everybody watching them... as soon as shapeshit detected the trade attempt they blacklisted (some minor part passed though).
It is way easier to guide the "client" to obtain the moneros and sent to them, and then convert to whatever (BTC probably) without that many worries that the money gets blocked somewhere along the "laundering" process or not blocked but closely followed leading to their identification and arrest.
Why I am "complaining"? Because I think that when all those arguments about Bitcoin used by criminals are completely erradicated there will be less bumps in the road to moon.
We just need to see how price reacts EVERY SINGLE TIME a dark market, shaddy exchange, etc gets closed.