I mean it's not really a big mystery, I'm not sure where this discussion is going. A ton of super popular food is ridiculously unhealthy. Coca Cola is basically super-sweet water. Jams don't usually have added fats but they do have a ton of sugar. Cotton Candy is literally only sugar (and some colorant I suppose).

Clearly the incentives here are simple, you want to make a product that people like as much as possible while making it as cheaply as possible. Health is not really a factor, insofar as not too many people don't drop your product because they see it as unhealthy (ads will help with that, all Nutella ads I've seen in France focus on "giving energy to children for their activities" or some bullshit like that).

So that's how you end up with something that's mainly palm oil and sugar. The market has spoken. I'm sure that if they reverted back to their original recipe they'd lose a significant amount of clients.