Channel analysts believe that desktops make up 10% of Mac sales (5% iMac, 3% Mac Pro and 1% each Mac Studio and Mac mini). If anywhere near close to true, I am not surprised Apple skips a generation on the desktops as the demand is not there. I mean how many people replace their Mac desktops every three years, much less every year? It has to be a fraction of a fraction of a percentage.
Apple wanted to be "first!" to 3nm at scale on phone/tablet/PC and were willing to accept a "substandard" process to achieve it, but only for product that could sell enough to make it worth the while. And that meant the iPhone, iPad and MacBook families.
I personally believe Apple never planned to offer M3 on the Studio and Mac Pro and therefore the UltraFusion connector was omitted from the M3 MAX because there would never be an M3 ULTRA. I would not at all be surprised if the M4 MAX has the UltraFusion connector back so two of them can be paired to form an Ultra, though I would prefer Apple develop a custom Ultra SoC going forward to better utilize all those cores and memory.