Because SCION is mostly said as a joke in the more serious carrier world.

SCION is practically speaking proprietary, and has 1 and maybe a half implementations. I have a laundry list of real problems with SCION but SCION feels like one of those entities that would get quite legal-ey if discussed publicly.

Because BGP works, is understood, and has been debugged by thousands of people and billions of sessions between dozens or hundreds of implementations.

So the benefit of changing out all that infrastucture needs to be much higher than the cost.