So why did they not even try Haiku 5.5?
Why would Amazon go to another company to let them provide an adjusted decider model?
Amazon already has all these tickets labeld, they could just have build it themselves?
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.