Whenever a new version of Apple’s Mac operating system is released, more old hardware (especially devices no longer for sale) are not supported by the new software. Likewise, new hardware is released with specific features incompatible with older versions of software. Unfortunaely, newer models of Mac mini are incompatible with older versions of macOS.
In our case, the Late 2014 version was originally released with OS X 10.10 and therefore does not support OS X 10.9 or older. In this situation, to make sure our customers have access to a wider selection of operating systems on all available hardware, we typically install VMware ESXi on the Late 2014 Mac mini and create a virtual machine running OS X 10.9 (or older, as requested) and allow it to use all of the system’s resources.
While the existence of the VMware hypervisor will cause the machine to be slightly slower than a machine running OS X directly on the system, it does allow users access to a larger number of operating systems on the latest Mac hardware.
More details about which Mac operating systems are supported on which Apple hardware can be found on Apple’s support website.