Atmospheric drag effects rocket performance less and less the larger a rocket gets (square-cube law), so that a rocket like the Space Shuttle can effectively ignore the atmosphere. Conversely, the smaller a rocket gets, the harder and harder launch gets. This is the problem Rocket Lab has decided to attack.
A decade ago, SpaceX launched the very similar Falcon 1. Falcon 1 was designed a decade ago, and you can see a large amount of the tech advancements in the last decade in Rocket Lab's design:
1. Cheap, lightweight Lithium Polymer (LiPo) batteries.
2. Cheap and performant brushless motor controllers.
3. Additive manufacturing for cheaply making complex parts without significant labor costs.
4. Advances in composites to enable the first orbital-class composite-bodied rocket.
Without any one of these advances, Rocket Lab would not be viable. It still may end up not being viable. Rocket Lab can never compete on bulk orbital cargo pricing. However, Rocket Lab's vision of applying recent tech advancements to bring down the smallest possible launcher size is extremely commendable.