Motherboard firmware is a fickle thing: in order to support as many CPUs as possible, especially in the multi-core and turbo era, each processor has to have its own entry in the support table in the BIOS. With luck, some processor variants can share entries, but ultimately the more processors a platform supports, the larger the BIOS firmware tends to be. As we have moved from basic BIOS implementations to complex UEFI interfaces, the amount of free space inside BIOS firmware has reduced drastically.
As a result, in recent generations, we’re seeing motherboards drop support for certain ranges of processors. Recently, a range of AMD AM4 motherboards have dropped support for the earliest A-series processors that were supported on the platform, mostly because these parts were not widely used at the end of the day. Now, Intel is doing the same thing: for users who invested in the X299 platform, in order to support the new Cascade Lake-X processors, that same BIOS version is unlikely so support the quad-core Kaby Lake-X processors.