Lots of factors.
For one, the rise of the "big three" in the US was the results of people not having much choice. That's changing by the day. Not only are other sports and activities available to watch but you can now watch pretty much watch anything from around the globe.
Also, I really hate when people use the word "dying" when numbers decline. More likely, it's just normalizing to a new baseline.
I also don't believe rule changes have much to do with anything. Those have always happened. All the big sports have slowly changed over time. People complain but they still watch. If being softer turned people off that much then Competitive Flopping (Soccer) would play to empty stadiums.
Many sports also seem to be foolishly fighting against the migration of many people away from cable/dish to internet viewing. Look at how popular the Olympics were when they actually got streaming functional. Where do I watch American Football? Yeah, I watch an illegal stream from the UK, which is the same place I watch the WWE.
