Part of the reason in the past was because DCA was not drawing as many people and it was to save on operating costs. Then with the new California Adventure last year, they needed to change that for awhile to accommodate the new interest in the park. If that continues to hold true or not years into this upgrade, we will have to see.
But here is another reason for the time of closure offset that you aren't considering: it's about merchandise sales! When California adventure closes, one of 3 possible things occur: 1) guests just go to their cars and leave (at least the ones without a park hopper), 2) they go back to the Magic Kingdom for an hour or two (depending on the closing time for Disneyland on that day), or 3) they linger along Downtown Disney for awhile. In two of those scenarios Disney is banking on someone spending MORE money! Ice cream for everyone; some popcorn; that Peter Pan hat you saw earlier in the day; a watch from the watch shop on Main Street USA, or just some souvenirs junk for the kids. It's about concentrating the "point of sale" into a smaller area which is going to be open anyway for a few hours.
It's all calculated to cut operating costs AND generate more sales.
BTW, many people aren't old enough to remember, but years ago Disneyland had very limited days of operation during the winter. During some months there were whole days or groups of days midweek that the park was not open.