CDR on Nashor's Tooth?

I play this game for about 2.5 years now, but I never got the point, why the CDR on this item is a unique passive.. All other items giving 20% have it as a normal bonus.

I don't want to complain about it, I'm just curious, why that is. It's not like it has that many other stats that would make double Nashors a viable build path on pretty much any champ at all (except maybe Kayle or Diana). Can someone please explain that to me, if you know how come its CDR was decided to be a unique passive?