Most people (hackers included) still don't know that Bitcoin is not anonymous and can be traced.
And they need a mean to get funds, a mean that is accessible to the weakling that wants to pay the ransom.
If they ask for a coin that cannot be bought in most countries directly for fiat, it's #FAIL. And this makes the list very short, excluding (from what I know) all the anonymous coins.
And then why not just stick to Bitcoin?