It's mostly due to personal preferences, I noticed (and got told) ages ago that I prefer certain things, certain character traits, types, etc so you can easily end up with the same characters again and again simply due to what you personally prefer.
Using dices or some other randomizer you can increase the variety of characters (and things and events if need be), and give the challenge to write different characters. Just have to stay true to the results and not alter them because you don't like X trait. Of course, character growth is a different thing since that is gradual progress within the story.
