It was purely based on their in-ring performance and it was universally judged on the number of quality matches, how effective they were at their role whether it was babyface or heel, longevity, ability to get over in various promotions, selling, offense etc.
With Rey they basically had his entire career on tape from 1989 - 2016 and they enjoyed how versatile he was. He could adapt in any environment he was in, whether it was Mexico, Japan, ECW, WCW, WWE and even back to the indies.
Meanwhile Austin missed a lot of time due to injuries and even when he came back had to adjust his style to take less bumps. Considering the obvious drawbacks, he actually did really well in that company to rank as highly as he did.
With Hogan, while he couldn't hope to touch the top performers of all-time he was extremely effective at working his own style of match and his efforts against guys like the Warrior demonstrated how sound his psychology was. He also showed in Japan that he could adapt to the environment. While the Hollywood Hogan era delivered some of his worst outings, the fact he was able to go from the biggest babyface to the biggest heel was another tick.
I forget what happened to The Rock. I think it was a case where he was put head to head with Hogan, and because Hogan had more runs on the board, The Rock just didn't get enough votes. He was one of those guys who was said to appear on a lot of lists lower down in the 70s, 80s and 90s which cost him. Basically they didn't see him as a good worker until the 2000s and even then a lot of his matches were plagued by interference. Personally I don't see how you can watch Hogan vs. The Rock at Wrestlemania 18 and not find a place for him. He's certainly on my list.
