Some wonder why the fuck Mario would give up his castle (you know, the bitchin one that he had in Super Mario Land 2). I did too.
I had a "eureka!" moment, though, which became one of my favorite Mario fan theories ever. And it also explains why Wario got a castle all of a sudden.
Here's the general gist of it, though: It still exists, but it's not his anymore. Mario and Luigi decided they'd be more comfortable inside their normal house (it reminded them more of their modest home in Brooklyn), and they formally moved in a week or so after the adventure, abandoning their old castle completely. And it was forgotten and just collected dust... until Mr Wario Wario himself came back to the castle for old times sake. When he noticed it was empty and nobody lived in it (meaning it's not theft), he got his stuff, moved into the abandoned castle, and slowly remodeled it into the castle seen in games like Wario World.
Nobody else remembers the castle. It's Wario's little secret, and it's all his. The only other person who knows about this is Waluigi (who half-lives in the castle; Wario considers the castle Waluigi's as well, and he lets him hang in the castle when he needs a place to sleep or is simply lonely, but Waluigi usually prefers a more modest living space much like the Mario Bros.)
TLDR: Mario's old castle and Wario's Castle are one in the same. The latter is just the former several years in the future after Wario took the castle for good.