Why do many people play Candy Crush without ever buying anything while some spend all their money on it? I'm trying to understand how free-to-play games like Candy Crush Saga use existing knowledge of the human brain to keep people engaged and how they convince players to pay money for in-game goods. Some of their game mechanics remind me of the Skinner box. Interestingly, some sources claim that a very small percentage of players contribute 50% of their revenue. I'm assuming this is because not everyone is as susceptible to their monetization tricks. Some addicted players dump all their money into the game, which puts pressure on game developers to protect vulnerable customers from themselves.

Research on big spenders in free-to-play games seems to be scarce, but are there any parallels with existing research (e.g. on gambling) that explain why many people will never spend a dime on these games while others are struggling not to spend their life savings on it? For example, is there a significant difference in how individuals respond to conditioning?
The economic concept of marginalism comes to mind. Actors will spend money to buy virtual goods if the marginal cost (price) is lower than the marginal (subjective) utility of the virtual good. If you have lots of money it's easier to spend $10 on extra lives. This explains some of it.

Most of the purchases are small, so the individual cost/utility analysis is biased, it's "just" $10. Over time it might become a problem.

It's a lot like any other kind of addiction, and I think (baseless speculation) that the "big whales" are more likely to be predisposed for addictions than the general population. The utility, positive affect generated by a feeling of progress and accomplishment, becomes harder to maintain and creates a "need" to keep playing. And when the individual transaction costs are so low it's that much harder to resist.

Edit: I've found some sources that might answer your question better.

This paper discusses how game designers might think about it. Not peer-reviewed.

From a different behavioural economics perspective.

Some other papers on the topic.

Note: Most of these discuss microtransactions and virtual goods. Candy crush is mostly lives, leading to different motivations than games like Team Fortress 2 with cosmetics like hats and skins (and keys).