Security is the big one. C++ DLLs have relatively free reign and are difficult to sandbox. With WASM you have a much stricter security model where the host program has full control over what APIs it has access to.

The addons are developed by third parties that aren't Microsoft, so there's a serious risk of malware and other ways of getting the user pwned.

The added future-proofing/portability is a nice bonus, but I suspect maybe not the main motivator.

Particularly for something designed to run on consoles sandboxing of addons is basically a requirement. Microsoft are never going to allow random people on the internet to run unsandboxed code on the Xbox because that's a fast route to people jailbreaking the DRM on them. I suspect the fact this also limits the use of Flight Sim mods as a vector for malware on Windows is just a happy side effect.

The real security for add ons would be to use external processes with OS IPC, even if it is more resource intensive.