With only few EPUB3 files around, device producers have little reason to increase EPUB3 support, with little EPUB3 support, ebook creators are not incentivized to make EPUB3s by sometimes taking advantage of its advanced features.
As EPUB3 to some extend is backward compatible and improves several basic features (specifying the cover explicitly, footnotes, no duplication of file reference), you could in general switch to EPUB3 without using multimedia or scripting, or at least provide EPUB3 and EPUB2 at the same time.
Anyway, in case of interest see
this and that (
1/
2).