I think it's more to do with compliance concerns with the EU's Digital Markets Act (DMA). The DMA enforces specific regulations on digital services to promote fair competition and interoperability. Which is good.. BUT that means that Apple cannot ensure the security of all applications running on an iPhone... I think iPhone Mirroring under these regulations might compromise user privacy and security - say if a bad actor app from a non-apple market were installed on your phone (similar to what happens if you jailbreak).