Three reasons:

• Most people who read the beta version know why I specifically want chapter 4 to be released on a Friday. It was serendipitous and unplanned when I released the beta on a Friday evening, and I'm trying to recapture that in the real release. Making sure this happened was a design goal of my schedule.

• My second roomate tried to read the entire thing in one sitting, and complained that once things got heavy and philosophical, he felt a bit overwhelmed. I want people to have some time to mull things over and think "what would I do in that situation?" before they see what happens next.

• One of the Equestria Daily prereaders recommended that I do so. If I had written this chapter by chapter and posted as I went, I would get n chances on the new list on FIMFiction, but if I post the entire thing at once, I only get 1. Each time Friendship is Optimal is on the new list is a chance to introduce it to new readers. Posting in chunks corrects for a bias against writing and editing everything up front, since, IIUC, you get less attention than if you post as you go. Putting three days between releases is a trade off between not making people too antsy for the next chapter and trying to give other stories a chance on the new and featured lists.