Actually, it's because it contains some performance regressions. To quote their release notes:
"This release is not a typical beta, in a sense the stability is the same or better than 1.9 and can be used in production. It does however include a few performance regressions documented below that don’t allow us to label is as 2.0 final. (It also contains many performance improvements.)"
PyPy is generally so stable that people are using nightlies in production, provided they pass all the tests.