Gamasutra explains why Grim Dawn is successful:

How Grim Dawn carved a niche in the competitive action RPG genre

Crate Entertainment’s recently released Grim Dawn, an action-RPG in the style of Iron Lore’s 2006 hit Titan Quest, began development in the hands of former Titan Quest developers. After a successful Kickstarter campaign and Early Access period, the 1.0 version of Grim Dawn garnered solid reviews and over 400,000 sales, according to Steamspy.

At first glance, the trajectory of Diablo-inspired Grim Dawn looks like the now-familiar "spiritual successor" scenario, in which fans rally to fund a project that promises to get the band back together and recapture the magic of a classic PC game, even if it doesn't have access to the original property (see: Broken Age, Pillars of Eternity, etc.).

But Grim Dawn is not exactly like those other spiritual successors. As Arthur Bruno, CEO of Crate and former lead designer at Iron Lore, tells it, Grim Dawn’s development process began long before Kickstarter existed. And thanks to Titan Quest's unusual publishing deal with THQ, Grim Dawn was built on the same game engine as the title that inspired it.

Still, the success of Grim Dawn demonstrates why this model of game development works, and what conditions developers need to pay attention to if they want to succeed with a "spiritual successor."

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