Because it makes you and your horse unbalanced
To be on the correct lead will keep yourself and the horse balanced, however "counter-canter" (eg. The horse is on the left lead whilst going clockwise on the circle) can be deliberately asked by the rider the horse should bend with the poll to the outside, this can improve coordination and communication (this is different to the wrong lead because you have asked for it).
I hope this helps😊
it keeps the horse and rider more balanced.
In the posting trot it's better if there on the right diagonal it's easier for you to keep your bakence and rithum
When horses are on the wrong lead and you make a tight turn they will trip and fall.☺
It's a balance thing. When a horse is on the correct lead he can correctly balance himself around corners and durning circles. When they are on the correct lead it can make things very difficult for the horse.