Many breeds trim or shave manes to accentuate the neck.
Draft horses want to show off thick, beefy necks so their manes are pulled to be about 6" long the whole way down.
Friesians and Andalusians are breeds where a lot of hair is prized, so manes/forelocks and tails are kept braided up meticulously when not in the show ring and very little is done to the bridle paths.
The miniature horse is shown in halter similar to an Arabian, as that is one of the breeds considered "delicate and beautiful" which many people want to emulate. Therefore, they want to show off thin/hooky/long necks and a fluffy or long mane would hide that. Therefore, the mane is shaved so that it does not hide the neck.
However, miniature horses also have "long" manes and tails, and you will very rarely see a mane that is pulled short. People want it both ways... long but not obscuring the neck from either side that a judge may look at the horse.
So there you go, that's why. If you like long/fluffy manes, then leave them be. Just be forewarned you will have to have a really, really, really special horse if you want it to then go National Grand Champion in halter.
The length of the bridle path *should* have something to do with your horses' neck set. The mane should fall straight down from the neck and not be able to be seen from the other side. If your horse has a lower neck set, this means a longer bridle path. A horse with a more upright neck can pull off a shorter bridle path because with its neck straight up in the air, the mane will fall straight down even from the ear. Most minis don't have necks like this, and so the bridle paths tend to be longer. It is all about optical illusions.
Andrea