WHAT IS TAKING AT&T SO LONG TO RELEASE THE MARSHMALLOW UPDATE FOR S6 EDGE+?

I am at wits end. First AT&T loads all their garbage on to my phone and doesn't allow me to delete it, then they troll online through their chat service and tell people to expect a release on 3/14. Now, nearly a week later, we have still gotten NOTHING. I will NEVER buy another carrier-controlled phone. I will either go back to iPhones or more likely go with a Nexus phone when I need to upgrade. You pay a little more up front, but over time it is about half as much and you don't have to put up with AT&T's B-S.