According to Intel, they didn't make Coffee Lake backwards compatible with older motherboards to prevent issues. This caused quite a lot of controversy among tech enthusiasts as a lot of people prefer the ability to upgrade to a newer CPU without having to buy a new motherboard too.
Getting Coffee Lake to run on motherboards with the Intel 100 or 200 series chipsets isn't easy because the chips are programmed not to run on older motherboards. However, a couple of modders managed to get Coffee Lake running on these motherboards and they even discovered methods to fix some stability and compatibility issues. You can learn more at Overclock.net.
CoffeeLake is now completely working on 100/200 series motherboards, so far Core i3 8100 has been tested working completely on the 100 series boards. Thanks to dsanke, littlehill, elisw and Mov AX, 0xDEAD from win-raid forums.
List of contributions done by the respective people:
LittleHill contributed in the adding of CoffeeLake CPU microcode and suggestions of using the correct ME Version to boot CPU
rootuser123 (me) fixed iGPU by extracting CoffeeLake iGPU VBIOS and using latest GOP Driver from SoniX and updating the VBIOS and GOP Driver.
dsanke contributed in booting 6 core CPU on MSI Z270 motherboard and helping along giving a lot of assistance in solving the problems.
elisw contributed in testing and writing guides
Mov AX, 0xDEAD solved the issue with the PCI-E x16 not working and CPU Voltage readout fixes when using CoffeeLake CPUs.