AVI was used for years, way before MP4 existed, and MP4 did not offer any relevant advantages so groups kept using AVI. MKV has builtin multiple-language-track

AVI was used for years, way before MP4 existed, and MP4 did not offer any relevant advantages so groups kept using AVI. MKV has builtin multiple-language-tracks, chapter and subtitle support (MP4 support is non-standard and incompatible), so if a release needed that MKV was used instead of AVI.