I think the difference here would be that the "point" of Google copying the API was not to make Oracle's Java be irrelevant, but rather to get a pool of developers that already knew the platform. Whether Oracle's Java stayed as popular or even got bigger was irrelevant for their purposes. The "point" of Microsoft's copying of the API was to make it so that eventually, people would only use their version, like they did with MS-DOS. The Supreme Court was considering the "point" when they were applying a test on "what kind of economic impact does the copying have on the original" and "does this act as a substitute for the original", which is on pages 32 and 33, where they're saying "Oracle / Sun tried to get into the smartphone space but couldn't hack it, Google entering that space didn't meaningfully affect them".
On the flip side, if Microsoft had been judged by the same standard, they might determine that Microsoft's reimplementation of an incompatible Java would have too much of an economic impact on the original to be fair use.
On the flip side, if Microsoft had been judged by the same standard, they might determine that Microsoft's reimplementation of an incompatible Java would have too much of an economic impact on the original to be fair use.