Actually, when you say ‘не прочитал’, it remains undefined if he started reading the article and just couldn’t finish it, or if he didn’t even start. So, technically, if you are trying to say that
Quote Originally Posted by lenssideways8 View Post
the action wasn't completed in the past because technically the action never actually took place
then you must use ‘не читал’.

The reason why even the native speakers struggle to see the difference between 2 variants is because ‘статья’ is considered to be something pretty short, so the possibility that someone couldn’t read it up to the end once started is rather small, thus the difference between ‘прочитал’ and ‘читал’ is also small.

For example there is merely no difference between ‘Я читал это стихотворение’ and ‘Я прочитал это стихотворение’.

But

If you changed ‘статья’ to ‘книга’, the difference would become more obvious.
Actually ‘Он вчера был занят, поэтому не прочитал эту книгу’ sounds like he really did read this book yesterday, but just didn’t finish it. So if you want to say that he didn't do any reading, then you must say ‘Он вчера был занят, поэтому не читал эту книгу’