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Im pretty sure the reason they don't go down all the way is because of the rear wheel well gets in the way of that. So, if thats the case I don't think there is a way to get around that.
on my jeep the wheel wells aren't in the way and they still dont go al the way down, so i dont think thats it. I'd have to say some type of safety reason, maybe bracing runs in that part of the doors.