Interesting question.
Now, I may be talking out of my ass, but out of everything I've read and all the research I've done, I think it boils down to a combination of running WWE like a company rather than just a wrestling promotion, and an aggressive business plan.
The WWE is not like other wrestling companies backstage. They are a publicly traded corporation. They have a team of people to keep the place running like a well-oiled machine. The backstage mentality is just...different. And I believe it's this that has helped the WWE thrive throughout the years. I mean, look at WCW, the company that should have succeeded. With people behind the scenes not knowing anything about running a company, it folded in the end after becoming a complete mess.
Something that could be examples of both WWE being run with a corporate mentality as well as them trying to stay ahead of the curve is the presence of the 90-day No Compete clause. The clause, which came as a result of Luger's jumping ship to WCW, is a way of preventing released WWE wrestlers from going to other televised companies within a sensible timeframe that would allow them to really capitalize on any momentum they may have gained from WWE. You see, Vince saw what happened with Hall, Nash, Luger, and others, and decided to act. This is part of the aggressive business plan I was alluding to earlier.
The other examples of the aggressive business plan are the buying of the territories, and the Attitude Era. The quickest way to get WWE exposed to a huge market would be to "invade". Vince swooped in, bought up all the local territories and their TV spots, and forced people to watch WWE's product. He forced it to become popular. People had no other alternative but to watch WWE should they want their wrestling fix. As for the Attitude Era, Vince saw that ratings were dipping for WWE and soaring for WCW, so he decided to act and put as much sex and violence on TV as possible. The TV he provided was aggressive, and shocking, and it caused people to tune in and continue to tune in to see what he could possibly get away with next.
Finally, this aggressive business plan has led to what we have dubbed the Youth Movement. Vince was too busy coasting on old stars recently that he forgot to build new ones. So what does he do? He starts to shove the youth into the spotlight, giving monster pushes to guys like Sheamus and Jack Swagger, in an attempt to build those new stars. While that worked for Sheamus, it failed for Swagger and McIntyre, and so Vince went and started this Youth Movement afresh, giving gradual and more believable pushes to guys like The Miz.
In conclusion, it's a combination of force and money that really has kept the WWE on top these years. They used the aggressive marketing to build the loyal fanbase, used the corporate mentality to keep the company chugging along, and it's worked for them through the years and doesn't show signs of stopping to work any time soon.