I brought this up earlier, but could it be that Classic Media chose to destroy their UPA/AIP elements when their rights expired? We all know Toho lacks U.S. elements to GRA, MvsG, Ghidrah and Godzilla's Revenge, and this is the first time they've been unavailable since the rights have reverted back to Toho.
I'm also curious if Toho has Sony's HD transfers of SON OF GODZILLA, SEA MONSTER, GIGAN, HEDORAH & MECHAGODZILLA as they let Kraken use 3 of them for their BRD releases. I'd be SOO happy if they at least used those for SoG & MG!!! The Sony transfers are so colorful and sharp, it'd be hard to watch Toho's transfers of them (even if they're in HD) after having the Sony/Kraken discs for so long.
I have a bootleg Blu-ray of KING KONG VS. GODZILLA and I can see why Criterion would choose to place it as a bonus feature as the "missing" scenes are very noticeably from a different source. I definitely was not expecting them to go the extra mile and license KKvsG from Universal/Toho, so kudos to them for making it happen.