The best remain theory, then, would be that the armor - especially the helmet - was made of a much more heat resistant material than the human body. Therefore, when set afire, the biological parts were burned to ash, but the the stronger armor remained in a deformed form. I still stay dumb move on the Luke's part to leave the parts laying around after the fact. Shoulda gathered them up and stored them or dumbed them in a lava pit or something. Nothing should have remained.
I imagined he took them to his new temple and made a crypt or a shrine or something. Kylo would have taken it from there. Don't know for sure though. Maybe the more Jedi thing is to not give a damn about broken technological remains and just leave it behind.