To be fair, he doesn't really have the stats in the Euroleague that would warrant a move to the NBA, especially when he is "already" 24 years old, so there is not much development to be expected. Scoring around 11 points per game and only 2.5 rebs and 1.5 assists per game doesn't really fill up the stat sheet, that's what NBA teams are looking for though.
It's as you said. Although Biberovic is a good player, he doesn't have the package that NBA teams want/demand. However, I don't believe that a player at his 24 years is done in terms of evolving and becoming better as years pass. He needs to work hard and play as much as possible, and while he can find this in Fener, the NBA will not give him the time or the space that he needs for this. It's a good move both for him and Fener, and I'm sure that his numbers will be better year by year.
I agree, a player that's 24 years old can still have some development in front of us, it's limited though. Players enter their prime at around 27 years old, before that there is time to shape their game and body. A player that is like 21 years old and puts him his numbers obviously would be FAR more interesting for a team in the NBA since he would have a much higher upside.
While Biberovic of course is a great player, he is on a different level skill and production wise cause European basketball is still way different compared to what's played in the NBA. If you want to make it there it's basically essential to make the move over to the states as early as possible. The longer you wait the smaller the window for success is becoming.