The reason is the following:
- One needs first to be released from the apayas at the Sotapanna stage. Then, release from the kama loka happens via the Sakadagami and Anagami stages. Finally, release from the rupa and arupa loka comes at the Arahant stage. It is always a sequential process.
- However, all those can happen while listening to a single discourse (or even a few verses in Ven. Bahiya’s case, which is extremely rare.) Also, those who make quick progress have done “much of the work” in their previous lives.