Before Boruto and Hinawari, the reason seemed to be a result of CONSUMING Biju chakra/ flesh. The reason Kin and Gin had them was because they physically ATE flesh from inside Kurama's stomach. The reason Naruto had them was because he CONSUMED nourishment from his mother who's chakra was mixed with Kurama's. This seemed to explain why hosts like Kushina and Mito did not have them, simply because they did not ingest the chakra, only hosted it.
But as I said, this was before the Boruto and Hinawari cases which seem to break the pattern. While the whiskers are less prominent in them, having only 2 on each side instead of 3, but they still have them. Since Hinata did not host the biju, it does not make sense that they would spawn whiskers unless Hinata somehow adopted nine tails chakra into her own body. Bottom line? It all made sense until Kishi broke the pattern with Boruto and Hinawari.