Just when I thought I could let this go...
No doubt in my mind that it was simply a budget cut. The maintenance cost in Disney's eyes probably outweighed the return they felt they were getting. Of course I completely disagree with them, but park attendance (which probably didn't drop off) probably made whoever made the decision feel like they were right.
Think about it: the castle just got lights in recent years, and the Osbourne lights have seen significant upgrades from previous technology that could be considered "obsolete" in today's LED world. Epcot could have followed suit and spent some money, but it was somehow determined that it wasn't needed.
Simple as that in my opinion, but also simply wrong.