Apple really doesn't even want to sell Mac Pros, but they have a small group of influential customers (probably in Hollywood) who want to buy them. They'd much rather sell you an iMac, because they're easier to support.
The rule about Apple towers for 20 years or so has been "you'll never see one that could hurt sales of any iMac". The cheapest tower has always been more expensive than the most expensive base iMac, and there has never been any configuration that is cheaper in a tower. If there's an iMac that can get there, it'll get there for less. The tower is only economical in configurations that nothing else can reach. When iMacs used laptop CPUs, there were towers with desktop CPUs, but now that you can get an iMac with an 8-core 9900K or even an 18-core Xeon, the tower has retreated into more exotic territory.
This is frustrating for photographers, who often prefer desktop or tower computers to use with our Eizo and NEC monitors, but whose performance needs are nicely handled by iMacs and even top-end MacBook Pros. We aren't the people Apple is trying to frustrate, though. Their goal is to frustrate gamers! Apple doesn't want to support often unstable gaming hardware, nor to provide the APIs games need, which are a major source of instability in Windows.
The best illustration of this is nearly 20 years old - Microsoft released the relatively stable Windows 2000, then the very buggy initial version of XP a year and a half later. Windows 2000 had essentially no game support at all beyond Solitaire and Minesweeper, while XP was pretty much 2000 with a better user interface and all the gaming APIs bolted on. It took four or five years and three service packs to get XP's stability to where 2000 was. That wasn't all games, there were driver problems as well, but a whole lot of it was games...
Apple has decided not to provide hardware that will attract gamers to the Mac, thereby washing their hands of all the support issues. Any midrange tower will attract gamers - GeForces will show up in any PCIe slot that isn't impractically expensive, and they'll demand support for them. Apple is very happy with their in-house AMD graphics driver, which is closer to a workstation driver than a gaming driver in stability, and either can't figure out how to write its equal for NVidia or simply don't want to put in the time. By releasing only gamer-frustrating hardware, Apple has forced this support-intensive market segment to Windows.
The other half of this strategy is to keep making iMacs more and more attractive to non-gaming audiences. I suspect we're getting the new display on the next iMac Pro, perhaps as early as this fall! While the pricing may seem not to work, they could actually do it.
The standalone Pro Display is probably a 50% margin item - Apple will take a margin that high on expensive accessories that don't sell a lot of copies. That gives them $2500 to get the parts at wholesale. $500 of trhat is probably the case, power supply, Thunderbolt controllers, etc., leaving $2000 or so as their cost for the panel.
The other key to how to fit the panel's cost in an iMac Pro is that Apple will sometimes take a low or even zero margin on the display (only - they take their customary margin on the rest of the computer) in an iMac at first. They use the same screen for years, and the cost always drops, but they make a splash by releasing an iMac with an impossibly expensive screen. They did this both with the original 27" iMac (a $1799 computer when their 27" Cinema display with no computer cost $1000 and nobody else sold a decent 27" display for less) and later with the 27" Retina iMac - any display with that resolution was selling for more than the whole iMac.
If they set out to make a $6000 iMac Pro with the new display, they might be willing to subtract the $2000 panel right off the top at zero margin, leaving $4000 for the rest of the computer. 40% of that $4000 is margin, so they can buy $2400 worth of components at wholesale - enough for a nice base configuration, and they can actually fit a little more in due to the Apple Tax.
Since iMac Pros are hard to expand, most customers buy overpriced RAM and storage from Apple (at 60% margin?). 64 GB of RAM and a 2 TB drive are $1000, while 128 GB of RAM alone is $2000. If the average customer buys $1500 in RAM and storage, they have an extra $300 from that 60% margin sale, allowing them $2700 in components at wholesale before the upgrades.