Huerter isnt going to be a top 10 pick next year. He likely won't be lottery next year etiher, but it's possible.
And again, the difference between the 10th overall pick in 2019 and the 21st overall pick in 2018 is only 6.5 million over the first 3 years. That is a lot, but getting to the 2nd contract sooner is more important than that difference. The risk of injury or just not getting picked that high (most sophomores who are projected first round dont raise their stock the next year), is not worth that extra money over the first contract.
But like I said, I doubt this decision is unilaterally money driven. If it was, entering now is an easy choice. Plus the teams who are attached to him right now (Celtics, Jazz, and Spurs) are all great landing spots for player development.