In between the 96-00 Olympics, everybody recognizes the name, buts its not a forefront thought. If you mention Michael Johnson to someone they will go, He is the guy the won the gold medals and broke records. Since Johnson is now retired, he really doesn't get mentioned much at all. Track and Field runners, and other competitors aren't considered mainstream, so they really only get brought up around the time the Olympics about to happen. People who don't keep up with Track and Field, may not even recognize the name at all.
Sampras outside of avid tennis fans, really dont get hte credit he deserves. Non Tennis fans would probably think Agassi was the best, even during his really down years before he was the more flamboyant and his face was plastered all over commercials during Pete's remarkable run.
With Golf being the being craze , everyone knows who Woods, so his name is known among all people, especially the astronimcal heights that golf has gone in terms of popularity over the last 10-15 years.
The main reason I think Armstrong is voted, is his story. I am not saying he isn't one of the greatest athletes, because he is one of the best, and by far the best endurance athlete, but I think because his cancer story hits home in so many household, he be considered the greatest just by winning 2 or 3 Tour De France. Every year the Tour De France will get 15-30 second blurb on Sportscenter on the day of each stage, and thats really all the mention, until the last couple of days, when Armstrong is in the lead, where they will interview guys like John Eustache (Hope thats the right name; bald guy), and Chris Carmichael, maybe Jim Ock., and ask about how amazing it is he can comeback from cancer and win. They don't ask how incredible is it to win x amount of TDFs in a row, its has to include the mention of cancer.