It isn’t, it’s a feature to keep the website relevant for a longer time. I’d be really happy to see this one break the mold, considering the rest of the development seems really reasonable and user-friendly.
That said, there has been consistent feedback that people want to play old puzzles, myself included. At least for me, the replay value is actually pretty solid after a few days' opportunity of forgetting the solution. So the next big feature I'm working on is the puzzle archive. Stay tuned!
I suspect this combo of a new daily puzzle and the complete archive will be the winning implementation over time.