Because the resident evil universe needs to be bigger and bigger, take for example the XMEN, they always bring more characters, just remember the first xmen, Cyclops, Jean Grey, the beast, the angel and Iceman, they will be cool, but then you will get more great characters, like Wolverine, Storm, Rogue, Nightcrawler, Psylocke, Emma Frost, and more, thats the essence of a timeless story, you need to add more and more characters, take for example Billy in the Rebbeca game, that was a cool character, the same with Kevin Ryman, that was a great character also or Carlos Oliviera more recent Sheva Alomar, the universe of characters needs to be bigger and bigger, besides some old character become weird, like Jill Valentine, which she lost her essence after resident evil 3, which I think that was her best performance in the saga, after that her design and personality going to nowhere, her new suit reminds me Sonya Blade and she lost her dramatic behavior of resident evil 1 and 3, opposite to Claire who actually was a good character in the 3d movie. Changes happen and new characters bring more pieces to the resident evil world.