React went from v0.14 to v15. The rationale was the library was stable at that point so they could switch to major versions. Why they went directly to version 15 rather than 1.0 is admittedly a bit odd. I guess it was marketing in the same vein of Java 1.4 to Java 5.
We didn't want a big hubbub around a "1.0" release, since we realized React was effectively already stable at that point and the changes from 14 to 15 were no more dramatic than from 13 to 14 or 12 to 13.
