Because they asked their patrons if they would prefer this and they said 'yes' because it might lead to more frequent updates. But, if each update takes as long as this one, then it could be a year or more for Day 12 to be completed.
There will be fewer events in each update than there has been in the half day updates, but if the 4 morning events balloon to over 1000 renders in total, that will take at least 4 months if not more. If however each part of the day is more like the first half of day 11, which was just 515 renders, it could take as little as 2 months as that's how long it took for that update to be done.
It really all depends on whether or not L&P will heed the call to adopt a "less is more" approach going forward.