Because it’s rude to yell. And it “blurs” whatever point it is he is trying to make.
If he had used all capitals, then that would have been yelling. He was simply making a point which came across to those of us who like to read! It meant that that part of the sentence was more important than the rest and for us to pay more attention to that section. I did not find it rude at all. Of course, I have more important things to be concerned about such as losing 20 family members in 38 months including my daughter, brother, husband, mother, aunt and the rest were cousins.