The vast majority of people bought the Mario games because they have fun gaming dynamics. Not a beloved relationship with the characters, fascinating places, etc. We must also keep in mind that the total games sold is biased because a great deal of Nintendo console sales came bundled with Mario titles, thus making Mario appear to have greater demand than reality. There is a number of factors that make Mario games enjoyable, but I wonder how many people actually care to go to a land. My brother has practically invested half his life (bit of exaggeration, but you get the idea) playing video games with a good chunk playing Mario. When I asked him whether he would want to go to "Nintendo land", he said "Why? I'll get far more enjoyment from playing the games, than experiencing them." He didn't say this about the HP or SW lands. No matter how familiar the characters or number of sales, I remain unconvinced the IP is well-suited for a theme park presence.
Also, many seem to be confusing recognizability with popularity. Mario isn't the most popular video game character anymore and hasn't been for a long time. He remains, easily, the most recognizable (and in the same league as characters such as Pac-Man).