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.