WannaCry and ransomware in general, often have no particular interest in trying to escape the various sandbox, because their first objective is the maximum possible spread, and you consider: how many people use running files in the sandbox as a precautionary measure? I believe a few of them, out of MT users and other advanced users.
Malware authors are working on large numbers, one million of targeted users, means at least hundreds of thousands of victims.
Malcoders are not worried if someone prevents the infection, being aware of that.
As usual, knowledge makes the difference.