Apple has enough incentive to do it now because Windows on x86 is something people actually want/need to use. It doesn't cost much to support and probably sells a bunch of Macs. Windows on ARM is not really something people would want to use, not to the enormous extent people want to to use Windows on x86.

Speaking from what people expect from a "Windows PC" - Windows on a current MacBook makes a decent Windows PC. Windows on a future MacBook makes for a very poor Windows PC that will make customers regret their purchase.

Apple currently uses standard PC architecture, with identical CPUs, GPUs and Wifi chips to standard PCs, and a (sort of) standard EFI booting mechanism. With ARM Macs, it won't. Clearly Apple in its hardware design is already not invested enough to support Windows - they could use standard EFI on ARM, for example. So why would they spend a lot of money to do it in software, so people can run a "crippled" version of a competitor's OS?

Note I don't actually believe Apple will actively lock this down. I do believe we will get native Linux on these things to some extent. But I do believe Apple will not lift a single finger to make alternative OSes happen on ARM Macs. Apple only cares about macOS and virtualization. I also think Windows on ARM is a great product, but I also know that common people that just want to use a Windows computer won't agree with me.