Language design often involves subjective tradeoffs. The author gives their rationale here: https://huml.io/specifications/v0-1-0/#why

Slightly off-topic, but I find the text of that anchor fragment amusing ("why? just why?") and a little disappointing. Automatic CMSs have robbed us of attention to detail - that anchor (for "Why `::`?") should probably be changed to `why-double-colon`.