They offer upgrades to people who are booked in categories that they think they can sell for more.
For example, if you are booked in a standard balcony that you booked 2 years ago and the ship has a lot of Spa or Havana balconies left, you stand a good chance of getting a good upgrade offer. This allows Carnival to get some more money out of you, and sell your old cabin to somebody who will pay more than you did.
I used to get a lot of upgrade offers years ago. Now that I book with a TA that explicitly doesn't participate, I don't get them anymore. But I also am not interested in upgrades so I don't care.