The Pro XDR, at this point, just isn't worth 5 999 US$. Maybe in 2019 when it came out, but definitely not now. If we assume that 1 299 US$ is a reasonable price for the Pro Art, then the XDR would be worth 1 999 US$ at best.
The XDR is a rather old monitor overdue for an update, but Apple doesn't lower its price, so it doesn't sell well, which means it's not a priority for Apple, which in turn means Apple won't invest in R&D for a Pro XDR 2, etc. In other words, Apple is going nowhere with the XDR. Meanwhile, the company isn't ashamed to do very incremental updates to its other product lines (compare, for example, iPad Air M2 and M3).