Why does xfinity prefer long-time customers leave, rather than offer a promotion?

I've been a long-time customer of xfinity (15 years). A few years ago we came across Xfinity mobile and loved the price and convenience. Both my wife and I now need a new phone. Xfinity currently offers a promotion to new customers for $400 off a new iphone, however, current customers do not get any promotional discount. Version on the other hand will give us 2 free phones to switch. I called Xfinity to try to see if I could simply get the promotional offer for the new phones and stick with them. They told me on the phone that they cannot do that and I may as well go with Verizon. How is it a good business practice to not try to keep your current customers that previously had no intention of leaving? I don't enjoy going through the effort of changing carriers but I can't deny a $2000 savings on phones from switching carriers when Xfinity won't even offer a dime to keep a 15 year customer. People are better off just switching every few years I guess.