Bit Tech had an interview with Andrew Wu, the product manager for the ASUS ROG lineup. A big chunk of the interview is about the Intel Coffee Lake-S processor and why it isn't compatible with Z270 motherboards.
Andrew explains the lack of backwards-compatibility was Intel's choice, he reveals it's not really a physical limitation. The change in power delivery makes a little bit of a difference, but he suggests this is not really relevant. The real problem appears to be that Intel locked Coffee Lake-S to be exclusively compatible with Z370 motherboards. It doesn't look like motherboard makers can do much about it as Intel is also using the chip's integrated Management Engine (ME) to prevent it from running on older boards:
bit-tech: So it’s not a physical limitation? Intel said it was to do with power delivery.
Andrew: Not really. It [the power delivery] makes a little bit of difference, but not much.
bit-tech: So what are they referring to – the 20 or so unused pins from before?
bit-tech: So if you wanted and Intel let you, you could make Z270 compatible?
Andrew: Yes, but you also require an upgrade from the ME [Management Engine] and a BIOS update. Intel somehow has locked the compatibility.