why? who put it there? lol it is only used for shojo's with Harem Genre but to distinguish that its males in there case. however here is only girls.

While it may seem sexist to distinguish between the two different kinds of harems the root of the word "harem" comes from that kind of culture.

Quote from wikipedia

Harem [...] "forbidden place; sacrosanct, sanctum", [...] "a sacred inviolable place; female members of the family" [...] refers to the sphere of women in what is usually a polygynous household and their enclosed quarters which are forbidden to men.

It would be interesting if "harem" as a genre could just mean "one person gets a lot of (romantic/erotic) attention from a lot of people" rather then that a man gets it (as in a gender neutral definition). But unfortunately I guess we are not there as a society yet. The want and the need to distinguish between whether the protagonist is female or male in these cases still exist. And since the root of the word is what it is the reverse is the reverse.

As for who invented the term or for that mater who put it on the site and why, I have no idea. I'm just trying to defend the term generally. 🙂

Btw is there a category/genre for bisexual harems?

well one or some of the girls do get attention interest from the MC and also from his brother(s), maybe that's why someone added reverse harem?

Since male rulers: having harems of women is a practice thousands of years old, it's a precise word with plenty of usage behind it.

Simpler answer: to make it easier to find manga with reverse harems.