Intel rushed Coffee Lake to the market ASAP because they wanted to stop the marketshare hemorrhaging from Ryzen 5 and 7. They didn't take bother take the time to built reserves and stock for a public release. I expect Coffee Lake SKUs to be difficult to obtain until Q1-Q2 2018. It also appears that current batch of 4-core 8 series chips are actually rebrand and repackaged Kaby Lake silicon (only work on 3xx series boards) not binned Coffee Lake silicon. It does easily explains their general availability versus nearly-unobtainable 6-core SKUs.
Gigabyte Z390 AORUS-PRO Coffee Lake R 9700K, 2x8GiB of G.Skill DDR4-3600, Sapphire RX Vega 64, Corsair CX-750M V2 and Fractal Define R4 (W)
Ivy Bridge 3570K, 2x4GiB of G.Skill RIPSAW DDR3-1600, Gigabyte Z77X-UD3H, Corsair CX-750M V2, and PC-7B