As people have mentioned that the most appropriate components for the iMac Pro are not available until much later in the year (especially the GPU), it creates problems if an updated iMac overlaps the performance too much. I could see Apple waiting until WWDC to release saying here is the iMac. We wish we could release a new iMac Pro, but these are the components it will have, but they are not ready until later in the year, when you will see the new model (they did an advance notice of the original iMac Pro at WWDC). If Apple also announce Arm Macs are on the way, this is one of the reasons why.
On the tower issue that has come up, I would absolutely love a mythical xMac. The Mini does not have enough power, the Pro is too expensive. There is a massive gap. The iMac is in that performance range, but it means updating the screen and computer at the same time, and has less flexible options. Some want a 32" screen, some 24", 27", etc. I like the screen on my iMac, but it doesn't have height adjustment (it is too high for me before anyone says put a riser underneath, the top of the screen should be at eye level for correct ergonomics), so isn't my first choice.
On the tower issue that has come up, I would absolutely love a mythical xMac. The Mini does not have enough power, the Pro is too expensive. There is a massive gap. The iMac is in that performance range, but it means updating the screen and computer at the same time, and has less flexible options. Some want a 32" screen, some 24", 27", etc. I like the screen on my iMac, but it doesn't have height adjustment (it is too high for me before anyone says put a riser underneath, the top of the screen should be at eye level for correct ergonomics), so isn't my first choice.