Why was the capital of the Western Roman Empire transitioned to Ravenna in the later centuries?

The capital was moved during the reign of Honorius at the beginning of the 400s. During this time Honorius had invested vast authority in his general Stilicho. The capital of the west was based in Milan. It had been moved away from Rome long ago to be closer to the frontier. Milan was well positioned because it allowed for easy access to both Danube and Rhineland frontiers.

During this time Stilicho had on and off relations with the Gothic general Alaric. In 401 Alaric had launched an invasion of Italy. The invasion directly threatened the capital. Concerned about the Emperor's safety, and therefore his own authority, Stilicho decided to have Honorius to Ravenna.

There are a few reasons that Ravenna was chosen as a good place to put the Emperor.

Defense - Ravenna is surrounded by marshlands. These marshes make it extremely difficult to try and take the city. Siege equipment is nearly impossible to use. Troops also would have a difficult time trying to forage for more supplies. Plus given the access points to the city, the city would be alerted to any enemy forces long before their arrival.
The fleet - The Western Imperial fleet was housed at Ravenna. If by some chance the city was endangered, Stilicho would be able to have the Emperor evacuated from the city to a place of his choosing. Furthermore information and supplies could also be quickly brought into the city.
Information - Access to the city was limited both in and out. Because of this, Stilicho was able to control the information that went in and out of the city. Stilicho could make sure that Honorius heard the things that he wanted. Plus the rest of the Empire would only hear what Stilicho wanted.

Please let me know if this helps at all.