It's because many people earn money by using less equipment than what they could (light armor has it's obvious advantages too, and not everybody uses a horse). People that want to play in heavy armor or with a horse either sell their loom points or are good traders with a good cash reserve (you need 1M to start trading seriously).
Honestly, there's no objective point in selling looms to upkeep a big armor. The advantages are good, but the speed and wpf reductions are still there. All in all it is personal taste. You like light armor ? Okay you will win money, be barely disadvantaged on the field, all in all earn much more gold/heirlooms and be advantaged by the autololbalance (you are considered as a peasant when you have light gear, switched to the winning team more often). You like heavy armor/horses ? You will loose money constantly, have a slight advantage that won't even make you win more often (maybe 0,05% more win rate...), likely have to sell your heirloom points to keep up and be switched to the loosing side because you are an evil powergamer.