alt Hacker NewsWhile the exact reason has never been documented, if you look at that mouse's design, you'll see that its first generation had a regular battery compartment on the bottom. When gen 2 arrived, they fully reused the same shell and only replaced that bottom part to now be an integrated battery with a charging port instead of a compartment for AAs. Moving the charging port would've required a brand new design, since every edge of the mouse tapers way too much for a port to be placed anywhere else. They would also probably need to change more of the internal structure, as opposed to just swapping a battery module and changing the bottom lid. In this case the constraint seems to just be about functionality and manufacturing. Apple has made many controversial design decisions that have no functional justifications in the past, yet people keep bringing up the mouse.
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While I get the feeling you appreciate the, erhm, efficiency with which Apple modified this product, the problem is that Apple is not supposed to be efficient. They don't need to save money on the engineering process because they are not hurting for money. They sell themselves as being a design-forward company that prides itself on making bold, not expedient, choices. To take a shortcut like that shows a lack of respect for the customer to whom they are charging premium prices for these items.
The reason people talk about the mouse is that it's one of the worst ideas they ever had.
At the time, I remember someone claimed that the reason was that they were afraid people could leave it plugged in for convenience. Apple thought that would lead to a worse experience because their mouse was designed to be used wirelessly. I think it was actually more related to aesthetic "icks" by the designers, because people would have disconnected the cable if it was in the way.