Intel currently finds itself in a pickle. The company is struggling hugely with its 10nm process, bad yields have resulted in years-long delays and even now there are rumors that we may not see volume production of Intel's 10nm chips in the second half of 2019.
Then again, Intel is still seeing very healthy demand for its current product lineup. But as a larger fraction of Intel's output is shifting to 14nm, the chip giant is having trouble to keep up with demand. We're seeing more and more ripple effects from the 10nm fallout, including rumors that Intel is outsourcing some chipset production to TSMC.
Now word is going around that the Z390 chipset may fall victim to the 14nm supply issues. German tech site ComputerBase picked up rumors that motherboard makers are preparing another wave of Z370 motherboards because the fate of the Z390 is uncertain. The Z370 is still made on the 22nm node, whereas the Z390 is designed for the 14nm process.
Basically, it seems Z370 will be the primary choice for the 9th Gen Core processors due to the 14nm capacity issues.