Why does Xfinity charge for unlimited data, when it's competition does not?

I have been looking at AT&T, Verizon and TMobile in my area, which are the only other options, and none of them charge a fee for unlimited data, so why does Xfinity?  I have no issues switching, and will be in the next few days, but you'd think Xfinity would want to keep customers they have had for 10+ years..