Iphone upgrade program Google Fi

I have an Iphone XS Max, purchased through the Apple Iphone Upgrade program. That all worked fine whilst I was an customer of AT&T for my cell service, but this year I moved to Google Fi.

Now Apple will not allow me to upgrade my Iphone any longer, since they say I MUST use one of the big 4 carriers (AT&T, Verizon, T-Mobile, Sprint).

Why is this, and when will Apple support Google Fi for the Iphone upgrade program? Also, why does Apple even care which carrier I use, as long as I pay them for the phone??

thanks!