It's a true open standard developed collaboratively by multiple competing top-tier companies, has multiple interoperable implementations, has been designed from the ground up for running untrusted code in a sandboxed environment, has been designed to be a general-purpose compilation target rather than with tight coupling to a specific source language, it has enough killer apps that is will likely continue to be developed and supported by these multiple companies, and it was developed with the hindsight of many of these previous failed or only partially successful efforts.

Some of these previous efforts might have one or two of these features, but none of them meet all of these criteria.