The reason is probably because in 2015 about 1000 Fiverr reviewers got sued by Amazon for creating product reviews alleged to be fake, so Fiverr probably doesn’t want to risk paid product reviews that might not be unbiased so probably has not allowed any paid reviews now.
Though technically I don’t think Fiverr lists in the terms of service that creating review gigs are not allowed. If it can get a terms of service warning it should really list there that they are not allowed if that’s the case. You could mention to them that it’s not listed there. It does list things like fraud not being allowed but if you were (or were going to) write honest, unbiased reviews there’s nothing written in the TOS that says that’s not allowed. If it’s against the terms of service of some other site (such as Amazon) that could also be a reason for it not to be allowed, but really that’s only if you were doing it so it could be put on that other site (eg. if you had written in the gig description that it could be for that other site).
It could be others have not been found yet by Fiverr. It might also depend on the wording of the gig. eg. if it’s being done just to help an author make sure their story is okay it should be okay. eg. “beta read your story/novel”, some might also have the word “review” in it and it might be okay (though maybe it’s risky) but it’s when it’s a paid review to be put on a site like Amazon as a review of the product that the issue would be.
So - was your gig to create reviews that could be put on places like Amazon or were you reviewing/checking a writer’s work to make sure there were no issues with it (and it wasn’t meant to be displayed on some site)? If it was the latter I assume that should be okay and maybe the wording of the gig was wrong ie. not clear enough to Fiverr that it wasn’t the former.