Submissive mares are generally unsuccessful in the dating scene.
Mares are naturally dominant, but there are a lot of those who would like to be a male's plaything. On the other hoof, stallions are naturally extremely submissive, and the ratio of submissive stallions to the dominant ones is extremely skewed towards the former, much more than the ratio of dominant to submissive mares, which is way more balanced, although the dommy ones prevail.
Subby mares are typically seen as beta, face shaming for wanting to submit to the inferior gender, and are occused of being lesbians by their more dommy peers who think they're pathetic lesbos with a shit taste in fetishes, and they give stallions the ick.
Subby mares are typically introverted and/or nerdy which doesn't help at all, but there's a word going around that a lot of successful and powerful mares are into maledom, even if you wouldn't suspect them of being into that at all.
Subby mares gain A LOT from the contact with humans, as men are typically dominant and would like to have a highly cute, noticeably smaller female as their fuckpet.
Similarly, submissive men also get a lot of extremely beautiful, both in personality and looks, 10/10 females willing to use them as their playthings, perceiving their submissive behavior towards them as attractive and natural, while they give ick to the majority of women.
And the there are switches from the both sides who just keep winning.