One of the big reasons that cruise lines are able to keep their trips so affordable is that they use tax loopholes. If you know a little bit about maritime shipping, then you're probably aware that a lot of shipping companies will register their boats in foreign countries in order to avoid having to pay U.S. taxes. The same is true of a lot for a lot of cruise companies.
That said, cruise costs might one day be a bit more expensive since, recently, many places have called for addressing this loophole. But for now, it's one of the main reasons cruises are so cheap.