Complex sentence: Hiromu Arakawa wanted to introduce Winry in Chapter 3 of the manga due to the lack of female characters in the first two chapters and because of her importance, but her editor told her it was too soon to do it.
Split into simpler sentences: Hiromu Arakawa wanted to introduce Winry in Chapter 3 of the manga due to the lack of female characters in the first two chapters. Her editor told her it was too soon to do it.