You're cherry-picking. The IMDB rating is for the Ultimate Edition, which those who already like it bought into it. And that's 35K reviews. The general audience review of the original movie was 686K reviewers and gave it a 6.4.
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The Metacritic audience score only has 7K reviewers. A drop in the bucket compared to the 686K of IMDB for the original cut.
Also, look at the critics' score...
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The Cinescore, which is a scientific sampling of the audience on the opening weekend gave it a B, which is pretty low score for a genre film. Usually, the opening weekend audience is on board with the genre and franchise. The audience for BvS, not so much.
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The B.O. fell 68% it's second week. That's not good word of mouth going on there.
And not to mention this:
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Critics hated it. The audience was mixed. It was not greatly appreciated by the general audience. It's no masterpiece.
Critics are awful. Who trusts them.
It's a case where you have lesser versions of films. Judge them on the better versions. Not studio interfered ones.