The politics is part of it but I think the bigger issue is the changing nature of entertainment.
First, regarding the politics, I don't think it's the message so much as it is the scripted, almost formulaic feel to it. Where "activism" in Hollywood once meant standing out and making waves against the establishment...it now seems to be all about blending in together, delivering the same message and saying "we ARE the establishment"..
The bigger issue, in my mind, is the erosion of two things upon which much of Hollywood greatness was founded: The Magic of film and the Cult of celebrity.
In short, the industry and the people in it simply aren't that big of a deal anymore...
Hollywood "star power" was already declining three decades ago as the public was allowed to get closer and closer looks at celebrities' personal lives. Today we have "big name" stars communicating directly with fans on social media. Celebrities were once demi-gods...today they are publicly traded commodities
Meanwhile, we are probably less than a decade away from a group of unknowns producing a feature-length film on their iPhones and iMacs and receiving massive amounts of critical acclaim in the media.
New generations of watchers have been brought up believing that entertainment media belongs to THEM...not to a particular industry, producer or artist. The declining Oscar ratings, I believe, are simply a reflection of this attitude.