My guess would be it is because the (Dragon) scroll we use in the main quest is the one that specifies the time for the prophecy. The (Blood) and (Sun) scrolls explain how, while the (Dragon) explains when. After all, if this prophecy came to bear during Morrowind(game), we'd have to hunt Auriel's Bow in Vvardenfell, which would be useless to Vyrthur. By 4E201, Auriel's Bow, a crucial part of the prophecy, had ended up in the old Temple to Auriel and thus served Vyrthur's goals. And Vyrthur says he had a part in creating that prophecy.
Or, more likely, they wanted to tie the main quest to the DLC main quests. Which makes a certain amount of sense, since we're playing a Dragonborn individual. Likely the first Dragonborn we have played. And the Dragonborn are pretty important figures in Tamriel's mythos.