Simmons said as much when he was asked about that topic. He supported his argument by saying that Dallas has been just a bit better (emphasis on just a bit) than the Atlanta Hawks, who have his presumed runner-up choice, Trae Young, on their roster.
Previously Simmons had picked the exact same guy when questioned on the exact same subject at the All-Star Break:
Both Doncic (who holds averages of 21.1 points, 7.6 rebounds, and 5.9 assists per game), and Young (who holds averages of 18.9 points, 3.6 rebounds, and 8.0 assists per game) are considered frontrunners for the honor, along with Suns center DeAndre Ayton (who holds averages of 16.3 points, 10.3 rebounds, and 1.8 assists per game).
Doncic also has seven triple-doubles which sits beneath only Simmons (12) and Oscar Robertson (26) for the third most by a rookie in NBA history.