I believe it’s a limitation of the memory controller on the M4 chip. It can only address 32 gb of ram. Addressing more ram would require more die space
Apple's on their fourth generation of in-house processors for Macs. By now, the pattern is quite clear: They have several tiers of processor. Not all products offer all tiers of chip. The iMac and MacBook Air have never been offered with anything other than the base M-whatever chip, and the Mac Studio has only been offered with the Max and Ultra processors.
