Well, the futanari tag is older than 1other, and we never really discussed the move to 1other seriously for one reason or the other.
Note that 1other right now is supposed to be for characters for which the gender cannot be determined, whether they're in full armor, facing away from behind or in a very wide shot. With futanari you can usually tell theyre girls, only with a dick attached, hence why they've been historically tagged as 1girl.
There's also the risk that by tagging futanari with 1other the 1other tag will be overrun by futanari, which is much different than the currently intended usage of that tag.
Adding futanari means 1other will mean "characters for which the gender cannot be determined, and girls with a dick".