You have to look at the Sorcerer class as a whole to understand why Blazing Starfall makes sense. For example, on my Sorc I built for a one-off I have Arcane Spellfury (+1 to hit after landing an At-Will), and then Rimestorm and Poisonous Exhalation. Both of those attack FORT, and they give the targets a penalty to their FORT defense. In other words, it allows you to set up combos where you hit with an At-Will, then Rimestorm, then Poisonous Exhalation, then back to a FORT targeting At-Will.
Blazing Starfall is the same way. The idea is to drop it on some enemies, punish anyone that leaves the zone, but maily to keep them grouped for the followup attack. With Arcane Spellfury, you'll also get a +1 on that attack roll since you hit with an At-Will.
Does it outdamage Scorching Burst? Probably. Does it outcontrol Scorching Burst? Probably. But we're talking about an At-Will here, and you're comparing it to an At-Will (Scorching Burst) that the designers have flat out said was underpowered.
A better comparison would be to look at Grasping Shards (Invoker At-Will) versus Blazing Starfall. Grasping Shards is low damage in comparison to a Striker At-Will, but that's okay. I would also say it's the more "controller-like" power of the two. Sure they both try to keep you in the area, but Blazing Starfall does so just by doing a little bit of damage, whereas Grasping Shards actually slows them. A monster with a lot of HP can escape the Blazing Starfall zone easily and do whatever he was going to do on his turn, but you can't escape more than two squares because of the slow effect unless you burn extra actions. That's why Grasping Shards is the more "controller-like" of the two.