There is no real lore reason but just pragmatism - it's a currency connected to high level gear so by cycling the currency keeps players doing new content to get the 'new shinies' (which results in the old tomes being devalued accordingly and thus removed).

Poetics tomestones however are used for several different types of content that remain static so hence the currency remained intact (such as items needed for the ARR relic upgrading quest).

There's no actual lore behind it, anything mentioned by the npcs are just padding to make it interesting.