Mostly a platform you can find programmers for. I believe that's really the main driver. If you have business logic in TPF, you need TPF literate 360 Assembler developers. Technically, you can program in C, but legacy integration and performance issues make that of limited use.

People moving away usually tried to extract the business logic such that the TPF layer was mostly a distributed NoSQL store. That bought a lot of time, but it seems like the skill shortage is now hitting that layer.