My assumption is that when designing the Model S, it wasn't yet clear that charging speeds would eventually become good enough that it made battery swap unnecessary.

Tesla wrote the playbook for a modern EV, so it's easy to look back in retrospect and assume they had a single clear vision, but battery swapping could easily have looked like a credible path back then.

Although I'm pretty sure that by the time they publicly announced it they knew it was a dead end, and its only use was publicity and ZEV credits.