While there are differences between Clang and GCC language extensions, most of them are compatible or at least can be worked around with some relatively trivial

While there are differences between Clang and GCC language extensions, most of them are compatible or at least can be worked around with some relatively trivial hacks. Compare this with trying to support MSVC, which supports none of the extensions and has a poor track record of getting even the standard stuff straight (especially in the C, not C++, compiler). By dropping MSVC, you gain a lot of useful language extensions that work with Clang and GCC. For example you get portable SIMD, atomic operations and cache prefetching and a lot of other __builtin features which are unambiguously useful. The alternative is to support MSVC but then have to maintain some kind of middleware layer to work around compiler incompatibilities. Since the platforms supported by MSVC are already covered by other compilers, you don't gain a lot.