Sonnet 4.6 was released 6 months before Luna (and Luna's major price cut came a month or so after that IIRC). So it's likely they started with Sonnet long before Luna was an option. I assume they evaluated Sonnet 4.6 vs Haiku at that time and found Haiku insufficient.

After that, I'm guessing it was just fairly low priority to update a working system. Amazon's net income last quarter was >$20B. Saving millions is huge on the scale of one or two engineers spending a week tackling it, but it's too small for the organization to make it a big initiative. By the time engineers decided to update it, decision models like Jev were popular and are naturally a better fit for the task than an LLM.