The GPIOs, GPU, booting process, etc. is all specific to it, and I guess manufacturing a compatible processor would need complicated if not almost impossible reverse engineering and might violate Broadcom's copyright or some contracts.
And a lot of commercial products, programs and libraries depend on the Raspberry Pi, so they have to be able to continue making compatible products.
My understanding is that the BCM2835/BCM2836 production run was dedicated to RasPi, although originally designed for OEM cellphone manufactures.
The figure I heard was that an outsourced SoC costs around $10-20m in licensing and NRE. That is basically go to a company that already has produced SoCs (eg. Samsung) and write a check. Considering that RasPi has sold over 5m units, getting another custom Soc made is not out of reach.
That seems to say that theres not a technical issue with making an rpi-compatible SBC but a political one.