Why do I have to wait ~24 hours between completing challenges?

I'm a little bit miffed that even though the event ends in 18 hours, I cannot complete the last challenge because I have to wait 23 and a half hours before I can start the next challenge. Feels really crappy to miss a few days and then not be able to complete the challenge.

I understand staggering the challenges, but wouldn't it be better if the challenges themselves unlocked on certain days rather than after the player beats the previous challenge? Has there ever been an explanation by the devs?

Now I'm not going to be able to get my beach balls :(