It's unexplainable to me. I was a decent kicker at that time. I was an 18 year old construction worker, pretty fit, flexible and accurate enough. I was probably a blue belt at the time. It was a solid kick just below his chest. They acted like it didn't even happen. I wasn't raised to question authority much, so I didn't ask or say anything.It just sounds like a really strange situation, unless for example your kicks were on them when they were already out of bounds, or you were not kicking in the target area, or you were in a low-contact bracket and so high contact was not allowed. In my experience, there's plenty of mis-judging that goes on, but if you got such a blatant strike, then the two most likely explanations are that you didn't have a legal strike (something that you should have known going into the match) or that the judges were corrupt.
As an example, I've seen first-hand people DQ'd from TKD competitions because they grabbed the other person's chestguard and punched them in the face. Which is something they should have known when they signed up.
But one thing was clear to me, that opponent had zero chance of beating me in any real fight, yet he won. That just didn't make sense to me, and my interest dropped in TKD.