The problem is that it's hard to use as a primary language because of a lack of direct Web API access. So basically right now it's not a casual decision to say "I prefer X over JS, so I'll just use that instead because it targets WASM!" Because there's this enormous cost of crossing that API barrier (which means you'll be using some JS regardless, directly or indirectly).

So for now the value-proposition only really makes sense for a fairly narrow subset of projects: mostly ones where client-side pure-compute is a bottleneck. Theoretically API access is being worked on, but I think it's still a ways out.