tl;dr
Until someone can explain the proof in something under 100s of hours, it's not considered proven. It's not useful until someone uses IUT as a tool.
It reminds me of other fields of science (like genetics) where many people have made a similar "discovery", but one person finally discovers and reports the results so clearly (e.g. Gregor Mendel) that everyone recognizes them as the discoverer... even though others came before (e.g. Imre Festetics) with similar concepts.