I believe it’s purely a marketing decision. The desktop market is small and is getting smaller. Apple has made the decision to emphasize the mobile market (iPhone, iPad, MacBook, watch, AVP) for the chip development. For Apple, the desktop market (iMac, mini, studio, pro, AppleTV) is primarily repackaging of mobile devices. Even the ultra chip is just a combination of multiple (2) laptop max chips.
Apple is basically decided the future market and business case is in the mobile market. That is where their development is concentrated. Desktops just get the hand me down from the mobile market and that’s “good enough” for the vast majority of the remaining shrinking market of desktop users.