The Carrier is great at long range bombardment of targets, however one thing is suffers from is the interceptors get shredded against any kind of local defense. For example, you can siege a Zerg base from a coast where a pack of Hydralisks can't reach the carrier, but they will quickly take out the interceptors making the Carrier quite useless.
Thus the Tempest was born. Being able to target units at extreme (siege) ranges does nicely take over the role of the carrier. However, to balance both the Tempest and the Carrier, the Tempest does significantly little damage for a unit of its cost and tech requirements. This is because it has the main advantage of range. The carrier however still does a good amount of damage but is more effective at closer ranges where the body of the carrier is a more attractive target than the interceptors. These two ships have very different roles as a result.
In the beta, the Tempest did a good amount of damage as an intended replacement for the carrier - however careful balancing has prevented any unit in the game from being removed.