Why?

In any case that's really irrelevant to the main point which is that Apple appears to have no objection to other OSes running on their laptops.

Though I wouldn't be surprised if they don't go out of their way to help and make it de facto impossible through secure boot systems.

Apple officially supported running Windows on Macs for many years.

What has changed that makes it so that Apple cannot officially endorse another OS nowadays when it clearly could before?

Boot Camp was officially supported. It gave them an advantage because it helped people move over to a Mac from windows because they could still run some of their software.

If Apple was to help Asahi officially, I think they would be worried that they would be on the hook either for continuing to support it or for damage done to the hardware by other peoples changes to the code.

Basically I think it’s a combination of not wanting to spend the money and being worried about the potential downsides of doing it.

The upside, in terms of additional sales, is likely very small.