The idea is to move realtime-ish stuff to a popular processor many people have worked with and moving the Linux stuff over to an appropriate platform.

When it came out we used STM32MP257 to demo a quadrature encoder debugger/logger (for internal use, not a product) since the IO supports easy interfacing of encoders. The micro's peripherals (SPI, I2C, GPIO etc.) are easy to program at the register level and you can run a simple mainloop+interrupts thing on the micro, then run Linux for your UI, networking, etc.

The fact that there's not much activity around these kinds of platforms (CPU+micro or CPU+FPGA) suggests it's too hard to combine the two in a way that makes sense for enough people, easier to just use standard components and have them side-by-side on the PCB.

It is nice to have them on the same die, though, for people like myself who like to work on both ends but don't know how to make their own PCBs.