I get the impression that a big part of it is just a general choice to be very conservative about forcing newer minimum versions. So to raise the version bar you have to make the positive case for why it's worthwhile -- merely "gcc 5 is ancient" doesn't suffice. The only reason it moved up from 4.9 is actual data-loss-provoking codegen bugs in 4.9 (see discussion in this lkml thread: https://lore.kernel.org/lkml/CAHk-=wjqGRXUp6KOdx-eHYEotGvY=a... )...
I do think they could move up a bit further, but it's useful to be able to just build kernels with the distro compiler and repology thinks that for instance Debian stretch (still an LTS supported version) is only gcc 6.3.
The distro kernel will stay on the LTS branch, certainly -- but the upstream kernel folk like people to be able to compile upstream kernels and not have to stick with the distro ones. (I've done it myself on occasion and i'm not a kernel dev; if I'd had to also get hold of a new gcc it would have been an irritating extra step.)
I do think they could move up a bit further, but it's useful to be able to just build kernels with the distro compiler and repology thinks that for instance Debian stretch (still an LTS supported version) is only gcc 6.3.