As to the reasons why Duelyst was open sourced, here is an interview published on PC Gamer:
“The truth is, to this day we still look back fondly at what our little game achieved, and reminisce with our community on Reddit and Discord,” Counterplay Games lead producer Richard Heyne told PC Gamer. “We are even following some Duelyst inspired games and projects! Through it all, one thought strongly resonated within our minds—people are still talking about Duelyst.
“We are experiencing some major changes in our industry: new technologies and consoles, and even massive consolidations that have occurred over the past couple of years. While only time will tell how these changes will alter the landscape, these shifts can potentially hit indie developers the hardest. That’s why we want to recognize that these developers, be they sole individuals or small teams, are vital for the growth and diversity of our industry.”
You can learn more about Duelyst and it’s being open sourced in the video below.