I bought the red iPhone 7plus from the Apple store (T-Mobile carrier) I want to switch carriers to boost but can't. Please explain why?

I bought the red iPhone 7plus from the Apple store (T-Mobile carrier) I want to switch carriers to boost but can't. Please explain why?

#1 the digital coding is different. It is like one speaks german and one speaks french. T-mobile is GSM which is compatible with most of the rest of the world. Sprint is CDMA. They own Boost.
#2 Because they said so. They make up their own rules as they go along, so you have to buy a new phone from them. A while back verizon would not let me use my previous contract phone on their pre-paid plans from the same company.
But after tmobile buys sprint it won't matter anyway

The phone would be locked to T-Mobile (GSM) so it would need to be unlocked to work on other GSM carriers let alone Boost Mobile (CDMA).

Not sure if that could be done.