Likely to be monetary reasons. DDG is basically (not entirely accurate but close enough) in the business of reselling Bing Search API and monetizing it through Bing ad network revenue (+some affiliate revenue through Amazon and Ebay).

If you also added Google results, your input costs essentially double. Also if so much of your marketing is based on bashing Google, it would be harder to justify such move from a branding perspective.

Forgive my ignorance, I really didn't know all of that. I only my criticism of google for the reasons I stated, they have grown so much that they don't care about their users and their search quality has been deteriorating quite rapidly. In part it'snot their fault as an army of marketers found their way to play the game to push their products up in results but, it's not only that. Google has been acting more and more like a corporate monopoly, they're not what they were when they started for sure