There's more to it than just the sandbox security model. The JVM bytecode doesn't have pointers which has significant performance ramifications for any language with native pointers. This limitation was one of the reasons why the JVM was never a serious compilation target platform for low-level languages like C/C++.

E.g. Adobe compiled their Photoshop C++ code to WASM but not to the JVM to run in a Java JRE nor the Java web applet. Sure, one can twist a Java byte array to act as a flat address space and then "emulate" pointers to C/C++ but this extra layer of indirection which reduces performance wasn't something software companies with C/C++ codebases were interested in. Even though the JVM was advertised as "WORA Write-Once-Run-Anywhere", commercial software companies never deployed their C/C++ apps to the JVM.

In contrast, the motivation for asm.js (predecessor to WASM) was to act as a reasonable and realistic compilation target for C/C++. (https://blog.mozilla.org/luke/2013/03/21/asm-js-in-firefox-n....)

So the WASM-vs-JVM story can't be simplified to "just security" or "just politics". There were actual different technical choices made in the WASM bytecode architecture to enable lower-level languages like C/C++. That's not to say the Sun Java team's technical choices for the JVM bytecode were "wrong"; they just used different assumptions for a different world.