He did not do it in 2015 because this trade wasn’t available to him at this price. The price was likely way higher. Value is key. If Towns was available for just an additional first rounder Hinkie is likely going for it.
The main goal of the process is to acquire superstars through the draft. There is no better bet for that than the first pick. But only if the price is right. At a certain point the picks you give up have a better chance in the aggregate of producing that superstar.
Also this team is way closer to contention than they were in 2015, the strategy evolves as their position on an expected win curve goes up. Hinkie would eventually have started positioning this team for contention and working to fill roster needs (ie Fultz) rather than simply acquiring assets.