I imagine because people that buy M3s over M4s do so for practicality.
People that buy an M4 typically do so because it looks better, and the Icons are part of that.
What was the cost at factory?
£1200.
I like both, if they were the same price I’d have probably got the M3, but it’s a significant premium (over £5k) when I don’t really need the extra practicality.
Looks are subjective, the rear arches of the M3 look great but coupes are generally considered sportier (but I do typically prefer saloons).
I also think the M3 name sounds better - more iconic, but the M4 is actually closer to the great M3’s of old, being a 2 door coupe.