I think people are reserved with Sheppard due to some subconscious biases. Let's face it, he doesn't look the part. He comes from London, KY, where he didn't play super-great competition in high school. But, he was super productive in college at the highest level. In a recruiting class of studs, he stood out.. He just turned 20 years old in June. He's a pretty elite shooter. And he proved he can play the PG position, which makes him increasingly more valuable.
Not everyone who comes into the NBA, even as a lottery pick, makes an immediate impact. Steph Curry, a similar style player, averaged 7.6 ppg across his first 8 NBA games. 11 ppg across his first 20 NBA games. But they allowed him to grow into his role and eventually finish 2nd in the ROY voting. And he was more than a year older than Sheppard as a rookie with two additional years of college.
*Disclaimer: this is no attempt to say Reed Sheppard is the next Steph Curry. The comparison is for illustration purposes only.