Maybe their goal was just surviving, not getting rich.

Also, you underestimate how trivial this 'one-in-a-million opportunity' is; it's definitely not a one-in-a-million! Almost anybody with basic coding ability and a few thousand dollars could pull off this hack. There are thousands of libraries which are essentially worthless with millions of downloads and the author who maintains is basically broke and barely uses their npm account anymore. Anybody could just buy those npm accounts under false pretenses for a couple of thousands and then do whatever they want with tens of thousands (or even hundreds of thousands) of compromised servers. The library author is legally within their rights to sell their digital assets and it's not their business what the acquirer does with them.