why do i have to be on level 60 class to get level 60 currency rewards

[MistakeNot]

Wow why exactly will there still be poetics? lol That is like ancient currency
As for the topic at hand everyone above has clarified the reasons SE did this

Ancient currency for ancient stuff. People will still want the old level 50 gear - for leveling and glamour if nothing else - so there need to be some way to purchase it. So why not use an existing currency?
I do wonder if poetics will really be used for the level 60 gear as well, or if that will be bought with tomestones of scripture(?) with some new currency introduced for the level 70 stuff?