The reality is that desktop computers don't sell as well, so whenever there's too much on the table for Apple's notoriously "lean" product design teams, you might see delays in updating them. Even when doing so seems like a wide-open slam dunk, e.g. putting M2 in the iMac or M3 in the mini.
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There's another reason why Notebooks sell more than desktops. They don't last as long. Their usage has a much higher abuse rate, they are more complex, they are more fragile and they are stuffed together. So people replace them more often, which means they are more profitable. No wonder computer makers like Notebooks.