Because the prophecy tells of a time when dragons return and that is what the Scroll is about...simple
Don't know what you need the Elder Scroll in Dawnguard for, but I can say that the Elder Scroll has nothing to do with the prophecy of the dragons returning in the main quest. It takes you back to the defeat of Alduin, which is history at the time the prophecy of Alduin's Wall is created, not prophecy.
If you're talking about the lyrics to the Song of the Dragonborn, they go:
And the Scrolls have foretold
of black wings in the cold,
that when brothers wage war come unfurled!
Alduin, Bane of Kings,
ancient shadow unbound,
with a hunger to swallow the world!
Note it's Scroll
s, plural. So even if we're using this as the base as why we need this scroll for both quests, we're told specifically that multiple Scrolls have this prophecy, so why can't we pursue another?
Also, what do the dragons returning have to do with the vampires trying to blot out the sun? The two don't seem related at all.