Why are existing customers not able to purchase a new phone at a good price?

I became an Xfinity Mobile customer just over 2 years ago. I was on a 24 month payment plan for a Samsung A51 phone, which completely died about a year in. I have been using an old Iphone since then, and eventually paid off the Samsung.

Now I am looking to get a new phone through Xfinity Mobile on my existing XM line. I am told through the website that I am not eligible for an upgrade since my phone is paid off (not understanding why they don't consider the act of replacing a phone an upgrade, but whatever) but I can purchase a new phone on a new monthly payment plan instead.

When I go to purchase a phone, there are several promotions going on, which look great. Until you realize that you need to activate a new line to get the promotion, and as an existing customer I don't qualify for any promotion on any phones! 

I contacted Customer Service through the chat, to no avail. I'm about ready to switch to a different carrier, as they offer much better phone deals.