Ponies and humans are psychologically different. We're primates and, despite our randy chimp ancestors, some primates do develop monogamy and tend towards pair bonding, ourselves being among them. Equines on the other hand (or hoof, lol), are more comfortable in herds led by a mare and guarded by a stallion. This explains why all the leaders in Equestria are female-a mare as a leader is simply more calming to the Equine psyche. Braeburn is an exception, of course. This could also explain why the friendships between mares are so intense. Mares still form lifelong herds, even if their culture has mostly accepted monogamy. (This also explains why Moondancer had such a melt down when she felt she was being rejected in her attempt to form a herd.) However, polygamy still pops up as can be seen in Hard to Say Anything when a herd pursues Feather Bangs. Seriously, rewatch that episode. They aren't fighting over him. They want to share him.

Just a side note... Monogamy is compatible with polyamory, and our closest cousins phylogenetically are indeed polyamorous.

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Furthermore, not all human cultures are monogamous or monoamorous either. As I see it, our Western civilization adopted monogamous and monoamorous positions in part because of religion, but more importantly for fairly pragmatic economic and sociopolitical reasons. I don't see a biological necessity behind it, and this might change in the future.

Tl;dr "because horses."

I actually read a very good article on extrapolating horse behavior into fantasy or sci-fi "space horse" races a while back!

Polygamous Space Horses

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I remember reading about the mating dynamics of wild horse herds and it sounded like pure drama fuel. The high status mares want to maintain a monopoly on the high status stallions but the stallions want to breed as much as they can while also staying in the good graces of the top mares, and the lowly mares and stallions were sneaking around trying to get whatever they could.

My understanding of the biological evidence is that, compared to other mammals, people are in-between -- mostly monogamous, but also opportunistic under the right circumstances. In mammals with high levels of sexual competition the males tend to have large testicles compared to their body weight, and in species where there’s either strict monogamy or the males faces little competition then his testicles are relatively small. And if you graph out a comparative testicle size chart -- isn’t biology fun -- you see people fall between the two extremes.

If mares do share stallions, the stallions might have relatively large testicles due to potential sperm competition. And for a clop story, I guess that’s one way of explaining how they have such impressive loads and low refractory times.