Put simply, the culture has changed, and the cast members who had knowledge of proper upkeep of the parks per the original SOP - including the lighting - are either no longer there or have given up. The guys who changed those bulbs knew which ones went out quicker or stayed longer, and they could stay ahead of the curve. That knowledge isn't passed on anymore, nor does the management care. This is partially due to time...the original managers who came up the ranks and opened the park under the original regime have now retired or gone elsewhere, and the people replacing them come from other parks and organizations that don't care about the details and aren't trained in the Disney way. Unfortunately, the training programs with the equivalent of the knowledge that was imparted during the 70s/80s aren't in place any longer. The other factor is the almighty dollar. Management is now always looking to trim here and there, and there's just a point where when you keep trimming, it isn't possible to balance show quality any longer. Much of the mid-level management in the theme park industry goes from Disney to Universal to Sea World to Delaware North (and a few others) and back again. They end up bringing the bad habits of one to the other, and occasionally the good from one to the others. But in general, lowest common denominator wins.
Until the late 90s, maintenance schedules at Disney essentially completely refurbished the parks/resorts on a schedule every 3 years. This was a uniquely Disney thing, and it was expensive. At a certain point, someone made a choice that it wasn't worth the cost any longer, and things should be fixed as needed, instead of preventatively. Specifically regarding lighting, eventually the majority of the property will change over to LED in fixtures that allow for it. Specialty lighting will likely be changed as parts fail. The work has started, but it will take some time. For Grand Floridian, they'll likely just do a complete replacement of the tivoli lights, maybe when they next do a roof refurb. Items like this are generally thought of as projects that can be combined with a larger scope project, unless someone gets a bug to get it fixed or someone in the executive suite says something.