Upgrade phone

Why wont xfinity allow me to upgrade ONE line of my mobile plan....oh that right because I was a few days LATE on paying the internet bill 2 times in the past 12 months.   You get the money and still are telling me that there is nothing you can do to upgrade the phone for me.   with that Im moving away from Xfinity mobile off to another carrier so I dont have to deal with the incompetence of this company.  Its a crock of SH*&^ ....thanks for nothing xfinity!!