There are 2 types of games where people can spend money:
1- P2W: Pay to win. This is self explanatory. Sometimes these games are even stretched to a Pay-to-play format. Typically, to be on top of the food chain in this type of game, you will need LOTS of money, or, at least a good amount, and an insane number of hours online. Free players in this type of game are purposely meant to lose, unless the player has mad skills and lots of free time, and even then, competing to a high level is nearly impossible because game-changing items/gear (call them however you like) are only available to spenders. Games with PVP content tend to fall into this category, where your survival and advancement are greatly influenced by how advanced you are compared to other players.
2- Freemium: This game falls more into this category. Basically a Free-to-Play game, with minor advantages if you inject some funds. These advantages are not game-changers per say, all they do is make the player's game less complex and easier for him to advance to the end game.
All i'm saying is, without a true PVP aspect, the incentive to be "stronger" than others is not as present and will thus drive less people to spend money.
P.S. I left out the suscription-based games because these do not offer a choice to the player whether to inject some funds and how much. Most of these games, all players pay the same thing, so everyone's on equal footing in that sense.