Given the internet, software distribution is global but the extent of laws are regional or treaty based at best. If you exist outside of the extent of that law, it's pretty much free money. You don't have to market, there is little to no risk and you don't have to support any business infrastructure and honestly you don't even have to have a novel idea.
Does <i>just</i> writing malware pay that well? No. Malware is largely commoditized.<p>The real money is in running the entire cybercrime operation end to end, from infection to cash out.<p>You can make a lot of money writing malware if you are part of a group that wants to keep their tech in house and needs something custom.
Keep in mind there are other motivators outside of money. There is definitely a section of malware writers who are politically motivated for example. Then there are also those who do it just to see if they can.