This is because people are probably not watching Smart actually play, and are just rating each player based on pure box score stats.
In this case Smart's numbers (6.8 points, 2.6 rebounds, 1.2 assists, 1.2 steals, 31% FG, 24% 3PT) don't look especially impressive.
It's hard to present Smart's value in numbers because much of what he's been doing well has been the stuff that's fairly difficult to quantify. He's been playing harassing defence which has really caused grief for opposing players (e.g. Deron Williams) and he's shown toughness, work ethic, IQ and decision making skills that are beyond his years.
Many of his greatest talents right now are intangibles, at least until he gets further in to the season and starts to get more comfortable with NBA offense.
Right now the two box score numbers that really jump out are his rebounds (4.6 per 36 - excellent for a PG) and his steals (3.0 per 36 - elite a any position). His turnovers (2.1 per 36) are also pretty low for a rookie PG playing in his very first NBA games - a good sign that he's not making too many mistakes.