Not sure really, it might be a holdover from its original iteration - it had a 20yd range when max range was 30yds.
I personally think it’s just to normalise the spell between specs, as all 3 Shaman specs maintain the exact same interrupt, with the same range/cd/duration/etc.
Hex also shares the 30yd range, though I think that’s just a rule for some caster CCs, as many have a 30-35yd range, like Poly, Fear and all SPriest ccs.