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    The Java VM has not been code-frozen since its initial release. There have been many versions. While they might be related, they are all different implementations, and behaviors could from one version to another (sometimes intentionally, sometimes not). Commented Mar 17, 2020 at 2:06
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    For the same reasons that multiple operating systems exists - Windows is not the perfect platform for everything and everybody. Commented Mar 17, 2020 at 9:16
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    A better question might be why there aren't as many competing implementations of other languages like PHP and Python. Commented Mar 17, 2020 at 13:50
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    @Barmar For Python, there are indeed: besides CPython (reference implementation), there is PyPy, Jython, IronPython, only to name some. Commented Mar 17, 2020 at 14:07
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    @JonBentley Yes, but the OP also made an incorrect conclusion in the question. The OP's question was triggered by the "Different JVMs can run threads in profoundly different ways." statement from a book and then started wondering why there might be JVMs from other vendors. "From other vendors" isn't even relevant to the quoted statement. Commented Mar 17, 2020 at 21:57