Two reasons.
Firstly, when we started the 300s we weren't sure if it might cause problems for the 75 worder, ie people would be fatigued doing both, so a 10 day gap allowed members to get the 75 worder out of the way before posting in the 300s. In point of fact we'd probably have been OK, but we wanted to err on the side of caution.
Secondly, the Challenges aren't merely bits of fun, they're also learning exercises for those of us who want to take our writing to a professional level. We'd found with the 75s that there were (and still are) a lot of members who post in haste and repent at leisure, and/or post and don't think about the pieces to any great extent, so leaving all kinds of errors in their work. We wanted to impress upon the would-be-professionals that all work needs to be revised and edited if it's to made the best it can be, so those who rush to post are forced to wait and, we hope, they use the time to make such revisions/edits.