I think it might have something to do with bootstrapping. In order to build GCC 5.x and later, you need a working C++11 compiler, so it introduces is an additional step when bootstrapping from GCC 3.
That said, I would love to see Void Linux provide packages for GCC 6 or 7 at some point. I tried to build GCC 6.1 from sources, but I can't get GCC 6.1 compiled objects/archives to link, might be something to do with how Void's binutils have been configured, but I'm not sure.
Would be nice if someone knowledgeable could take a look into getting it working and pushed out to the xbps package list.