I can't honestly say I'm surprised by this at all. I'm sure they'll spin it as a technical design requirement...
There are a few possibilities:
1. The most optimistic one is: Coffee Lake actually supports PCIe 4.0. I know, too optimistic to expect Intel to be that on the ball, but if it's true then a new socket is OK since PCIe 4.0 is a major upgrade.
-- But #1 is too optimistic --
2. The second most optimistic: Coffee Lake is technically not "supported" on Z270 but it can basically work anyway and the "official" reason it's not compatible is for some power delivery requirements that are theoretically out of spec on older motherboards that just barely meet the minimum functionality threshold but are actually achievable on most real-world motherboards.
Remember that some guy from Asrock doesn't want to make people think they can just use an old motherboard when there's the opportunity to sell new ones.
3. The most likely one: the new motherboards are identical to the Z270 but with a slightly tweaked southbridge and Intel is just being Intel.