Look at it from this perspective:
These are phones. Say they are right in the middle of roll outs maybe 1/4 of the way done then someone reports that their phone got bricked then another person reports that there is some huge bug that makes the phone inoperable. What they can do at that point is stop rolling it out and re assess the software and make sure that these bugs aren't just user error. A slow roll out means more control if there are bugs rather than pushing it to everyone at the same time and bricking everyone's phone.