Why did the WSOP move away from the RIO?

There was/is a lot wrong with the Rio.

But the primary reason for the move was the sale of Rio, making it no longer part of Caesars, while WSOP is owned by Caesars. They also own the Horseshoe name so rebranding Ballys and moving there made sense (from a corporate perspective).

As 2RedCards said, Caesars sold the Rio. But the Rio was also a **** hole. It was a shell of what it once was. The only good thing about it, was the the parking was good (What I've heard locals and people from LA say). But the location was bad, the food options were bad, the rooms were run down. Really nothing good about it.

That Pho something place was good. It was closed last weekend.

In addition to the above, there’s a good deal more space in the new location. WSOP is always looking to expand.

All correct, the parking was good, everything else was bad basically, there were also other things like Legionnaires' disease outbreaks and employees stealing money from the rooms. The RIO poker room was also miles away from the WSOP. And various other things, basically overpriced low quality everything.
But the main reason was prolly that it was not on the effin' strip. Let's face it, the strip is what Vegas is all about, at least for short-time visitors. As a commercial venture, no matter if poker or whatever else, you want to attract random foot traffic and not just peeps in the know. And there is no better place than on the strip. Nobody will come to the RIO accidentally if they did not book a stay there, but on the strip that happens all the time...