It's not that no one believes it, the problem is that this particular possible proof of ABC conjecture came from 1000-1500 pages of dense, new math. As in, the proof writer invented some new math, and from that new math he claims he can prove ABC as a sort of side-project to his goal of furthering his new mathematical area. No one else can really understand his proof because it would take years to study the text behind it, but a few are trying and may have found a hole in his proof. I am not sure what the status is since the article you linked, but as with huge code bases I am pretty sure that there must be at least one error in those 1000-1500 pages of math.

To be slightly more accurate, it gives a simpler proof of Fermat's last theorem, and proofs to a bunch of other way more esoteric theorems.