Why it ends

Because a product that keeps you is a product that needs you not to get better.

Retention is a business model, not a feature: An app paid monthly makes more money the longer you need it. Nobody sets out to build something that keeps people stuck, and the incentive does it anyway, gradually, over a hundred small product decisions.
Streaks are the clearest example: A streak is a reason to open an app that has nothing to do with whether the app is helping. In a product about sleep it is worse than useless — anxiety about a number is the last thing anybody with insomnia needs at eleven at night.
So this one is priced once: One payment for the eight weeks. There is no monthly plan to move you onto at the end and no upsell in week eight. If it worked you are finished; if it did not, paying us for another month was not going to fix that.
What happens when it slips: It will, usually after a bad fortnight or a holiday. Week eight produces a written page telling you what to do about that, and it is yours to keep. You can also run the whole thing again and pay again, and about one in six people do.

And what it is not

Not therapy. It is a structured programme you run yourself.
Not a medical device, and not registered as one.
Not a treatment for a sleep disorder. If you snore heavily, stop breathing, or fall asleep in the daytime, see a doctor rather than downloading this.
Not a subscription. Eight weeks, one price, and it stops.
Not gamified. There are no streaks, no badges, and no notification designed to bring you back.
Not funded by advertising, and none of your data is sold or shared.