Why creating new heroes and dont use older (existing) ones?

[DrNewGod]

Here is my opinion of sometimes what happens.

Writer: Hey I want to do a sidekick for (Insert character name)

DC Editorial: Sure, here is a list of characters you could easily use.
Anima
Jade
Empress
Bombshell
Sparx
Vulcan
Aura
Aquamaria

Writer: you know, I really just want to create my own, give him a mystery backstory, integrate that into the book, sort of thing. I will call him, "Sidekick Man"

DC Editorial: Sure whatever.

2 years go by.

DC Editorial : Hey we need to cancel your book.

Writer: Sounds good, want me to kill Sidekick man off or will someone want to use him later?"

DC Editorial : Whatever

Writer: I'll leave him around then.

New Writer:: Hey I want to do a sidekick for (Insert character name)

DC Editorial: Sure, here is a list of characters you could easily use.
Anima
Jade
Empress
Bombshell
Sparx
Vulcan
Aura
Aquamaria
Sidekick Man

New Writer: you know, I really just want to create my own, give him a mystery backstory, integrate that into the book, sort of thing. I will call him, "Billy Bob"

DC Editorial : Sure whatever

There is this. Also, back in the day, I suspect putting a new writer on a title was more about stewarding the property while telling a story. The advent of Image comics changed the creator/publisher dynamic in such a way (IMO) that putting a new writer on a title is about telling the writer's story, and stewarding the property is an after-thought, if considered at all.