The first step to address this is to ask yourself this vital question...

Do I simply write or I communicate?

The way you communicate reveals everything about you. Words are the clothes your thoughts wear

What is the difference between writing and communication

Excluding the means that is through televisions, radio and audio visual devices, I am referring to it in probably a contextual term.

----> Writing in the sense am referring to is just a means of putting words together in a written form , more like the way a magician conjures. But the thing with magicians is most people simply come around hoping to grab the trick.
Most people write without communicating and that is why there is so much confusion in this world as there is too much noise and less understanding of that noise.
When we try to communicate without conviction, we simply write. Writing does not connote negativity in general, I am merely trying to pass a message across.
The greatest records ever kept were communicated in writing and not the opposite way around.

----> Communication on the other hand encompasses all, it provokes a thought process in us. It brings out the alpha in us which dominates mediocrity. The process exceeds just simply passing across information, it involves painting an accurate mental picture of what you want your audience to visualize from the bulls eye stand point.
Proper communication brings out the dominance in our words and sets us aside from others, it expresses even the things which in the words of someone I respect "are visible to the blind and are audible to the deaf"

Why do people often find it hard to communicate?

It should be obvious by now already, because people often times lack the "command" in the command of language. Written language is beyond words, it is a written art which carries both expression and emotion.
To express it and communicate it properly, you must feel what you write that is, relate with what you write and not just write because you feel the need to.
In this world, you either have the connection to what your post is all about or you don't.
Permit me to make this comparison with having an orgasm. In the sense that, it's either real or not, there is no faking it, neither is there any substitution for the real deal.

Let us try to communicate more, instead of just writing,

Image credit: 1-2

Thanks to @elyaque for the badge and @son-of-satire for the generic footer