Character chemistry (finale spoilers)

Sometimes writers/directors/show creators are lucky when they cast actors who exhibit a natural on-screen chemistry. Yet, sometimes they throw it away. Why? (Example: Scandal, although that story isn't finished just yet.}

I never ever felt any on-screen chemistry between Annie and Ryan. Likewise, none between Auggie and Natasha. Their whole story line was a real stretch. Why do show runners throw away the obvious just to create a surprise? Viewers don't want a negative surprise, especially between two characters they have come to love.

Maybe the direction the script took accounted for the series cancellation. I have waited until I watched all five seasons (thank you, Prime) before I commented on IMDB. But now, I must say, I hate the ending. Lame and cowardly.