Easy because 5.1 still hasn't been released and is still in testing by Google so motorola doesn't have possession of that yet. And I know because of the galaxy s5, moto x and HTC one all have had atleast one of their variants updated without issue.
Good points...except the part where they are both wrong. Check the front page of AC, 5.1 has been spotted in the wild. And it doesn't take much time on the forums for each of those phones to see that they have had some issues with lollipop.
Posted from my Droid Turbo, Kelly and Ozone