Optimal readability is attained with 60-70 characters. This is due to human psychology, and technology such as monitor size does not change it.

When lines are longer it becomes more difficult for your eyes to keep track of where the next line is. Furthermore, code is not formatted as paragraphs but as separate lines, so you end up wasting a lot of screen estate for those few long lines. Having a low maximum means it will be close to the average and a larger proportion of the screen is utilized.