The CPUs are available, but Apple could just be waiting on suitable AMD GPUs from their Vega range before refreshing. June this year represents 12 months since the 2017 models came out and it seems likely to me that WWDC would be a good time to refresh the iMacs if at the same time as the new Modular Mac Pro and Apple Displays (and Mac Mini, of course!).
Edit: Probably should add that availability of motherboard chipsets might be a factor too. iMac uses modest chipsets which I believe aren't available yet. Coffee Lake CPU purchasers have to buy 'top of the range' Z370 motherboards at the moment making a complete PC fairly expensive.
