Historically, cryptolocker varieties have created a bitcoin wallet to be paid into, per infection, making it possible to identify those who have paid the rasnom.
If it all goes into one pot, how do they know who has paid and who hasnt? answer, they dont. There never was any intention to unfuck these infections for the victims
Honestly, this whole wannacry saga is the work of USA / UK unintelligence agencies and if you haven't figured that out yet, you need to look to history.