The reason Project Spark failed is the same reason Neverwinter Nights 2 failed.
Ultimately the editor was too complex for the layman to produce something meaningful.
Anyone who could actually create a balanced, well-thought-out, plot-rich game worthy of playing....might as well just use standalone engines like Unity and create a new, monetized game that wouldn't be destined for obscurity on a failing service.
I still have Project Spark downloaded on my computer and I've tried a lot of the "Hall of Fame" modules...they tend to be painfully mediocre at best.
No game can thrive like this. There's a reason why they didn't bother with a sequel to Neverwinter Nights 2.
As others have said, I quite enjoyed Team Dakota's standalone adventures Void Storm and Conker's Big Reunion. I really wish they would have expanded those into full-fledged, single-player games and not even bothered with the whole creation tool nonsense. It's just not worth it.
Microsoft tends to have these huge ambitions for their games, which ultimately leads to their downfall. First Quantum Break, then Fable Legends, now Project Spark.....I'm afraid Sea of Thieves is next on the chopping block, folks.