What? Where did you hear this? The SR556 / SR556E is an entirely different platform than the Mini14...
Think about it. Ruger certainly must have before offering an AR. There are thousands of Ruger fans out there who are quite loyal. Ruger had a popular 5.56/.223 semiauto rifle selling for around $800. Did they want to chance cannibalizing existing sales of a popular model with another $800 5.56/.223 semiauto rifle ... or position the new rifle at a higher price point where they'd have less canceling effect on each other. Obviously Ruger chose the second path.
The problem with the Ruger AR at $1200 or so, they compete with extremely good ARs at the same price point. Rifles made by Colt, BCM, Daniel Defense and others. And of course you can buy a very good quality AR at $800 such as PSA.
We can argue piston vs. DI or chrome lined barrel vs. non chromed or muzzle heavy vs. lightweight but in the end it comes down to Ruger vs. quality ARs at the same price or even hundreds less. Even though I have three Rugers I couldn't go with their AR.