There are a million RPGs out there, and they use a ton of different dice models for their main mechanics. D&D uses the d20, many use only d6s (Blades in the Dark, PbtA), some use only d10s (Warhammer, White Wolf)… but only one passes what I call “the T-shirt test”… that is, only one mechanic is popular enough to be put on a t-shirt, and that’s the d20. Why is that?