I’m not sure either. There are some technical differences. WASM is more low-level than JVM bytecode, which is important for some applications and makes it a more normal compiler target. It’s more integrated with the rest of the browser: it doesn’t come with its own rectangle like Flash and Java did, but instead manipulates the DOM just like (or at the moment, only through) JavaScript. And it’s a true open standard, not a corporate-controlled platform.
Do these differences really matter, or is it mostly just the timing?
WASM is definitely more low-level than JVM byte code right now... but I'm not sure it'll remain that way given the many proposals that are very likely to be implemented in the future (even though it's taking a long time for even simple ones, like multi-value returns, to get into the standard). For example, with WASI and GC, which will finally enable direct access to the DOM, how much lower-level will WASM be compared to JVM? Not much, I would say.
The big difference is that WASM, from the beginning, is supported by all the browsers natively (a consequence of it not being a proprietary technology, but an open standard), not as a plugin... If Java had started that way, the story would've turned out quite differently (but we know that at the time, the browser everyone was using was made by Microsoft, and Sun was a competitor so this would've never happened).
Do these differences really matter, or is it mostly just the timing?