Perhaps to allow for more replayability, after the player has completed the level with the species of ant it was meant to be completed with, the player can choose a different ant species to play the level with.
This is unlikely to happen - as much as having to record a whole host of new dialogue for other species, the campaign levels tell a story about an individual species which is then introduced to the formicairum. As we add more levels this will feel more like a naturally fleshed-out campaign.
I do agree that there's a gap in being able to choose your own species and play with it - and that should be filled with Freeplay mode.