As I understand it (and I may not understand it well, as I don't do this for a living), SoC vendors are primarily in the business of selling hardware, rather than software. Everything that follows is second-hand information.
Often, folks who want an SDK for that SoC get the hardware, a bunch of blobs to run the hardware, and a particular version of an OS to run the blobs and any ancillary software. Given that the vendor is a hardware manufacturer, you're not infrequently forever stuck with whatever blobs and version of the OS you were handed. (Unless -of course- you can do the dev work on the OS or blobs yourself.) This lack of focus by the vendor on the software side of things implies that -say- interesting new graphics APIs are only available on the newest hardware. Even if the SoC vendor does do the work to port said API to their older hardware, the folks who used that hardware may not want or be able to verify their software with the new stuff from the vendor.