I guess what they say is true about the writing reflecting the characters.
Kind of like the earlier seasons of M*A*S*H, when the writers pretty much made Hawkeye the main character of the show, while Trapper was reduced to being a sidekick, which resulted in Wayne Rogers eventually leaving the show.
But then thing of it is, when you have ensemble series like these, it's difficult to make sure you write equally for each character, especially considering that different people are going to relate to different characters, and of course, different people who are into different couples.