Japan had quite a history in their Super heroes aka Tokusatsu 特撮 and Monsters aka Kaiju 怪獣 series. One of their successful formula was to introduce new characters right after one of the character and series coming to an end. Such were the Ultraman, Kamen Rider/ Masked Rider and the Sentai genera aka Power Rangers. As the years goes by, the list of the characters grew.
To have a new character to out shine the older series was challenging itself. As the marathon goes on to introduce new character in the new series, it lost some of the fans along the way. When the rating hit a all time low, another formula would be come into play. This formula would be to revive the old characters and put them into play with new one.
This formula is not new to you. You seen it not too long ago in Kamen Rider Decade
In Ultraman Ultra Galaxy Legend where the whole Ultraman gang appear
Lastly the Sentai genera who have access to the powers of all the previous 34 incarnations of power rangers.
Kaizoku Sentai Gokaiger: Super Sentai 35th... by Koragg
Due to the old fans who has a connection to the old characters, it draw them back to the show. Kinda feel like a sucker right?
These formula appear to be the current trend in most of the Japanese product marketing. Don't be surprise you see such all stars in the coming days on other products especially with long history of characters.
You have been warned. :)