Probably because it has the 18+ setting. In general you can get away with a lot more on YouTube by actively setting your content to 18+... the downside is you can kiss monetization goodbye and, even if was monetized... it wouldn't matter because they don't actively push or promote the video at all.
In this case though it looks like it was flagged and then YouTube themselves added the 18+ setting.
All that said YouTube can be fickle when it comes to enforcing certain aspects of their ToS and there's a number of factors they'll generally consider, like the NUMBER of videos you have pertaining to the subject, the nature of the channel itself, how much TEXT is involved, etc, etc.
If you want something with a vag, here's an 18+ video I recently uploaded to my channel...