I gather it's greed. I recall some ex-conman saying that greed is the thing that makes most cons work, that even smart people will deceive themselves under its influence.
Well, I would not state that "those people" are immoral or stupid. Even academics or highly moral people fell for these scams. The big problem is that "those people" are blind to the signs of the scam, which seem obvious to people who know the matter - in this case the blockchain and the crypto-world.
In case of OneCoin, people were told they could earn some money by simply buying some fancy, technical crypto-thinghy. It's just the human nature to react positively to possible contracts or actions which likely return something positive for them.
This is the same issue for all other "scams", i. e. like some insurance-salespersons, which obviously sell overpriced or needless contracts, just to earn more money with their victims. Or car mechanics who tell the car owner the suspension arm needs to be replaced even if it's in perfect condition.
It's always the same: some greedy, immoral people exploiting the inexperience or missing knowledge or other people.
Not being stupid is no defense against being conned. On the contrary, a truly stupid mark isn't going to understand the scheme; an intelligent person will immediately see the way they stand to benefit. It's the willingness to cheat that gets you in trouble.
You need to be not stupid stupid, but reading these articles it is pretty clear that the marks are not "smart" if they're investing in something they don't understand.
The truly intelligent person will see the way the other person stands to benefit. That's how you avoid getting scammed. Unless you truly believe some stranger is offering you this great opportunity out of the goodness of their heart (in which case you have a thing or two to learn about people in general).
And if I can't understand how the other person benefits? I'm immediately suspicious.
This is not really a factor. A good scam involves the scammer and the mark going in on something together. What the scammer (ostensibly) gets out of the scheme is part and parcel of the pitch.