[AerieC]

Thoughts can be abstract, but when you're trying to paint a picture with words, abstractions can make your "picture" weaker. "Mortality rose" is a confusing statement. What does it mean? Did more people die, or did humanity's inherent mortality somehow increase? If you meant "more people died", then say it. You're not talking about the abstract concept of mortality, you're talking about people dying. In the time it took your reader to figure out what you meant, you may have already lost him or her.

Often it's a case of writers trying to be overly clever with their wordplay. For example, you wouldn't want to say something like, "her posterior condition was greatly exaggerated," when what you meant was, "she has a big ass". Sure, the first sentence is "clever", but it's not very clear. The second sentence is right to the point; there's no mistaking the meaning, and it paints a better picture in one's mind.

It's similar to active voice vs. passive voice. "Dan hit the ball," is clear and to the point. "The ball was hit by Dan," paints a weaker picture, even if it says the same thing. Why is "ball" the subject of the sentence. Are we supposed to care about this ball? Aren't we talking about Dan here?

Clarity in writing is important. Readers don't like to be confused. Anything that breaks the "flow" should be avoided, unless that is your goal (e.g. to stop the reader and make them think).