As she was Tar-Aldarion’s only child, he altered the laws of succession to allow his daughter to assume the throne after him, instead of her male cousin Soronto - with the requirement that any ruling queen be married, so as to ensure that she didn’t die childless (why this wasn’t required of the men, I’m really not sure.)
... which got me thinking. I love the story of Aldarion and Erendis, but somehow this particular question never occurred to me. So I spent a bit of time teasing out some possible answers to this question. Why would Numenoreans insist a ruling-queen get married for the heirs when this didn't concern them with kings? The most obvious answer is that's probably not their concern at all.
So do check it out:
1. AskMiddleEarth's original piece, well worth reading on its own</a>
2. my answer to this question