They just didn't say. I think we can safely assume they are not completely stupid. They probably tested a few models a few months ago and have been using Sonnet since as it passed their benchmark/performed the best and then didn't change the system since, because it worked.

The blog post was focused on the switch to a decider model, not the initial architecture.

I think I'd be foolish to 'why didn't they just?', on the basis of <blank> because no mention was made in the post.