While Mario and Luigi were in Brooklyn, Bowser took over the entire Mushroom Kingdom. They came back a decade or two later, shortly before Super Mario Bros., wherein they took back most of the land, which had been totally populated by Koopas. Mario and Luigi each set up shop in castles in strategic locations across the land, where they held the land down until the Mushroom Kingdom's citizens repopulated the area. After about a year, Mario left some housesitters and escorted Daisy, who had also been in Brooklyn, back to Sarasaland. Wario wanted to get back at Mario for never letting him be the sheriff when they played Cowboys and Indians, so he hired Tatanga to distract Mario while Wario took his castle. Once Mario had gotten rid of Tatanga, and Herman Smirch, he came back and kicked Wario out of the castle. Wario then founded WarioWare, Inc. primarily for the purpose of making his own version of Nintendo's Sheriff, so he could be the sheriff every time.
As for that giant robot statue land of Mario... I like to think of it as a metaphor for something. Mario did some weird soul-searching on the trip, and had to confront his own greed, represented by the three pigs in his brain, before he could face Wario.
...Sorry.
