They never had to, whether they had the best store or not. In market share and in public perception, Steam is considered the only game in town.
And because it is so huge, it's not unreasonable to believe all but the largest devs releasing new stuff aren't doing much better than breaking even or just above. In the end, devs, especially new ones, need to recoup their costs and get at least some profit so they can continue making games. I imagine that's the scenario Ys Net were faced with here. An unfortunate scenario for sure, but one that ensures the game recoups most of its budget and gives even the glimmer of hope for a fourth installment.
I backed this game, and I care more about that than where and how I play this.
As I said in the last thread, this is the road PC gaming is headed down, like it or not. We're seeing it with TV streaming. Eventually these storefronts will be fractured as publishers and developers seek to retain as close to 100% of the cut as possible. If Epic weren't doing it, someone else would.