What does Mozilla use Spidermonkey instead of V8? Why did Apple create B3 JIT instead of just using Turbofan?
Competing implementations are a cornerstone of standards. Indeed in many domains it's a requirement for a spec to have multiple compliant implementations to be considered complete at all.
The older I get the more I disagree with this POV. All other things equal, a single open source implementation is superior to several ones. Several implementations lead to duplication of effort, both for those developing them, and more importantly for those developing for those implementations. Software has to be tested separately for each implementation, often with vendor-specific hacks.
Competing implementations are a cornerstone of standards. Indeed in many domains it's a requirement for a spec to have multiple compliant implementations to be considered complete at all.