According to IGN, the reason for the delay is that it requires additional development time. Also, Thomas Happ also revealed that delaying the game can avoid burnout and crunch by pushing the release date a bit.
Here’s an overview of Axiom Verge 2, via Thomas Happ Games:
Axiom Verge 2 is a 2D “metroidvania” action adventure. Though a sequel to Axiom Verge, is a completely new game: new characters, new abilities, new enemies, and a new world. You can play it before or after the original Axiom Verge.
The sole developer, Tom Happ, is responsible for all the music, art, game design, and programming in this game.
– Expansive non-linear exploration of a huge interconnected map.
– Hack any enemy with unique and interesting consequences for each.
– Launch a remote drone with its own separate abilities and behaviors.
– Speedrun Mode—play with a streamlined interface and additional features to facilitate speedrunning.
– Plus one other major feature players will be excited to discover on their own