I'm actually surprised that the archdemon showed itself so soon. As long as it remains hidden behind millions of darkspawn and a hundred miles away from the front lines, who can get at it to kill it anyways?
Cailan jumping when Duncan said so was probably a factor. Even if that army got mostly decimated, enough got out (namely the Warden and the treaties) to ensure that the next one could form in time to be useful. I also wonder if the Fifth Blight's location was a large factor in keeping it down. It started in the swampy far end of Ferelden - if you look at some analyses of the Deep Roads map, that's probably past the Dead Trenches. A darkspawn under Rivain or the Anderfels would have a LONG way to walk to get to the bulk of the horde. Some would get killed by Orzammar patrols on the way. The initial horde had to either wade through the frozen swampy wilds and the Hinterlands, face down with the Legion of the Dead, or take the long way around to pop up somewhere like Honnleath and then make the trek across half of Ferelden to meet up with its buddies. Some were likely being converted by the Architect or the Mother as well, and they might even have been fighting each other pre-Awakening. Some might just get lost. The Deep Roads are nasty, and I doubt darkspawn come with a built in GPS.
So there's a lot of factors, really. The approximate Blight schedule on the archdemon's end probably went something like:
Pre-Ostagar: lurk around in the Wilds. Gather army for glorious invasion of surface world.
Ostagar: Crush puny humans! Eeh, did two of those Wardens just escape? Oh well. What harm can they do?
Lothering: Gather more reinforcements.
1st treaty mission: Start marching up from the wilds. Destroy Lothering and kill someone's sibling to create appropriate backstory angst.
2nd treaty mission: ...this is a lot of open space to cover. Gonna need more darkspawn.
3rd/4th treaty mission: Gather Bigger Horde. Where the heck are all my minions? Well, we can conquer the south for now, at least.
5th: Finally, a proper army. It's pillaging time!
Landsmeet: hmm, the Wardens are spending an awful lot of time around this one place....
Final Battle: Let's destroy this city! Wait, is that guy jumping at me? Aaah get it off! *falls onto Fort Drakon* oh crap oh crap *is killed*