It shows them just fine -- the issue is that on iOS, only Apple Watches are permitted to perform notification "actions" (e.g., replying to a message, etc).

Recently, EU pressure might mean that Apple will open that ability up to non-Apple watches as well.

I think Apple lets third party watches show notifications but they're not allowed to take any action in response (eg having a snooze button when your alarm goes off or whatever). Next version of iOS will allow interaction for a single smart watch at a time in the EU. I think it's still unclear if Apple will enable that feature outside of where it's legally required.