You'll never see an endurance horse in polo wraps for obvious reasons. They travel through sand, mud, brush, tall grass, and water. Polo wraps would do more harm than good. You also won't see any fleece-lined boots for the same reason. And as a general rule, you won't see bell boots. Things may have changed in the ten years since I last competed, but we always considered bells boots a dangerous piece of tack.
Riding through the conditions that endurance horses travel through means that a lot of debris can get stuck under the boot and cause rubs and irritations. The boots can also get hung up on wayward sticks and other trail debris like nails, wire, or metal shards. I've actually had near misses with all of these.
Both Speedy and Izzy live in front bell boots. Speedy wears them because his stride in the back has gotten so much longer that he started whacking the sides of his front hooves with his back feet, causing painful bruises. Izzy is just so rambunctious in his paddock and during turn out that he was ripping off shoes left and right.
Just to be on the safe side, my boys now go in leg wraps on all four legs, and the boots are lined with cushioned fleece to ensure maximum safety and comfort. Sheesh.
I hope my dressage divas (ponies) know that they're being laughed at by their endurance cousins.