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.
What has changed that prevents Apple from officially endorsing another OS nowadays when it clearly was able to do so in the past?