Comment by gerikson
Since the start, each problem has 2 parts (2 "stars"). Part one sets up the problem, ensures you have parsed the input correctly, etc. After submitting the correct answer to that part, part 2 is revealed, which sometimes expands the proplem space, adds new limits, etc. Something that solves part 1 might be inadequate for part 2.
Yes, but nothing (theoretically) stops him from saying: "congratulations, you have solved part 1, wait until tomorrow for part 2".
I think either the author thinks people appreciate more the 2 stages challenge, than having one problem each day; or, more likely, the whole "infrastructure" is already prepared for 2 stages challenges per day. And changing that meant more work, eventually touching literally 10 y.o. code. The reason for the reduced days is exactly the lack of time. I assume he preferred to have 12 days, and modify as little as possible the old code. Having 1 stage per day maybe would have been possible at the expense of having less challenges, which again defeats the purpose.