An ugly argument doesn't persuade anybody. (Except maybe the person making it.) Some of its features are: disorganization; rambling; sloppiness; errors; poor word choice; mediocre writing; no focus; exaggeration; harshness. Thoughtful deciders eventually turn that channel off. It's too hard on the eyes and ears.
Persuasive advocacy on the other hand has an aesthetic: purposeful; compositionally unified; sharpened to the point; balanced; craftsmanship. Advocacy tuned into those qualities has a better chance of acceptance by people. And the most beautiful argument uses those tools so well that it doesn't reveal them. The tools are hidden, like a musical composition that's in harmony without having to explain the science of harmonics or a painting that uses color harmony without a dissertation on how to mix primary, secondary and complementary colors.
It's for the same reasons people of all times, ages and cultures like good music, great visual art, a well-prepared meal, a well-designed building or landscape. It goes way beyond law. If the argument tastes bitter, is full of weeds, hits the wrong notes and keys, sounds over-the-top, lacks a sound foundation, is broken or worn out... people who are thoughtful deciders won't buy it.