shows, the manufacturers of the product the show is based on, often control the show in many ways possible in order to sell more, here are some tropes associated with this effect.
In order to make the new character seem very cool, he/she first appears defeating strong foes, but when his/her toys are no longer being sold, they became easily defeated.
A great way to add another character to the toyline.
Characters whose toys need to be promoted are given more prominence in the story.
When the toyline is discontinued, so is the show.
Characters who sell lots of toys and currently have toys in stores aren't allowed to die.
Characters whose toys were discontinued, or never had toys to begin with, are prone to this.
Toys of the character are no longer in production, so this character is swiftly removed from the show.
In order to keep characters who don't have toys anymore away instead of killing them.
Killing a character in such media is not always meant as a way to sell toys, but if his toy goes back into production, he will come back from the dead no matter what.