No reason? Matthews is a far superior offensive player already, and while Bergeron is better on defence, it's not enough to make up the difference. Bergeron has had what, one ppg season? Last year. While the only time Matthews hasn't is in his rookie season. No one can even hold injuries against Matthews here since Bergeron is far more injury prone. Matthews is also the second best goalscorer in the league behind Ovechkin, who gets ranked so high because of his goal scoring ability.
That's not even counting linemates. A couple years ago Bergeron wasn't talked about as one of the best players in the world at all. A solid two way player, sure, but whatever offence he has now is a result of Pastrnak and Marchand flanking him. Overall still a great player, but he's quickly become one of the most overrated players on these boards the past couple years.