I believe it has been reported here that certain people at Disney (likely the lawyers) were paranoid about the moving rocks falling on people. Instead they're supposed to install the new explosion effects from Disneyland at some point in the future (when that is is unknown, should have done it during this refurb but apparently they didn't, even Paris is getting the new effects during its current refurb).
Even though the ride went decades without guests getting hit by falling scenery until relatively recently. What this really is is Disney not wanting to spend the extra time and money on the old preventative maintenance and safety checks they did up until the mid to late 90s.