Probably for the same reason why Google keeps removing customizable features from Chrome. The more of them you have, the harder it is to test the product due to all kinds of unforeseen interactions between them. Someone probably said, "there's 10'000 lines of 30-year-old window positioning code we have to maintain, let's replace them with 1'000 lines of fresh window positioning code, then we'll have more leeway to add more Copilot features into the Start menu".