Why no SLA with Specified Uptime?

After recently posting about instability issues, out of frustration, I started doing some research on ISPs. What makes ATT so fundamentally better, that they have the confidence to offer 99.9% uptime in their service level agreement, vs Xfinity's "best effort" at maximum uptime? Why can't Xfinity figure it out? Is it a difference in available resources, technical know-how, principle, or dedication?