Title says all why did they move from the Rio what was wrong with 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...

The restroom situation was beyond horrible. It is much better at Horseshoe/Paris.

RIO parking was really, really great. I also liked the vibe of all of the RIO poker rooms being so near to each other, versus the Paris and Horseshoe WSOP areas being so far from one another that it feels like two separate tournaments. That's about all I can come up with in favor of the RIO.

The Paris/Horseshoe food options are so much better than the RIO options that I can't do justice to the vast discrepancy. Plus, you also have the options to go off site during dinner breaks, something not very easy from the RIO. The lines to sign up for tournaments were much, much shorter at Paris. Part of that is a lot of people were using kiosks to sign up at Paris/Horseshoe, but the fact is there seemed to be a lot more people working the cages at the Paris as compared to the RIO which seemed understaffed to me. Being on the strip at Paris/Horseshoe just feels right, whereas RIO made you feel like you were on a desert island away from humanity. RIO felt grimy, whereas Paris/Horseshoe has a fresher feel.

I was dealt more premiums at the RIO than Paris/Horseshoe, but I suffered more bad beats at the RIO. I know, I know ... small sample size.

Not that I made any final tables at either, but the location of the TV final table set up in the Horseshoe gives a really unsatisfying vibe since people watching in the stands were almost nonexistent when I stopped by. Every time I wandered over to the final table, almost no one was watching. The couple final tables I checked out at the RIO seemed to have more energy. The Horseshoe room where the TV final tables (as well as other tourneys nearby) are seemed really quiet. Felt like a library to me.

I have a hotel preference other than either location, so I never stayed at Rio or Paris/Horseshoe. Everything I've heard is that RIO was not an elegant place for a room to sleep in.

One of my least favorite things about Vegas is Strip traffic, so Rio was fantastic for me. The food at Horseshoe is not much better than Rio. Paris is a little better. I mostly ate fast food anyway, and my favorite places were much easier to get to from Rio. If it was late night I could just head to the Orleans just a few blocks away.

I stayed at the Gold Coast when the WSOP was at the Rio. Breakfast at TGIF was nice. Subway right there as well as other food options. The rooms were comfortable and quiet. I once had a room at the Rio with a wide expanse of windows and the A/C had trouble keeping up with the June Vegas heat.

The Gold Coast was great. I used to go to that Subway three times a day lol.