It looks like they just recently (this month) dropped the lifetime license.
Huh, I just checked and you're right. Sandboxie is developed by just one guy. I can only assume the lifetime license wasn't providing him with enough money. It is his only source of income IIRC.
Edit: Straight from the developer himself:
To ensure future growth of Sandboxie, the licensing model shifts today from a lifetime licensing model to an annual licensing model.
Licenses issued in the past which did not include an expiration date will remain without expiration date, and may be used on more than one computer, per past licensing terms.
All new licenses issued starting today will expire in one year after initial activation of the product key, and are limited to one computer per one product key.