1. Terrible roster construction: During an era in which the league is trending towards lineups with smaller, athletic, multi positional players that eschew traditional big men, the Bulls doubled down on the latter. The result was an imbalanced roster where the Bulls' most valuable asset and most talented player, recent No. 7 pick Lauri Markkanen, was somehow third on the depth chart at his natural position (more on this later) while Paul Zipser, a guy who put up a 6.9 PER last year with negative value over a replacement player, was all but guaranteed to be part of the wing rotation. With no logjam in the first place, this fight never happens.
2. Inability to develop talent: For all the shit that Mirotic and Portis are getting, there's no question that the two have talent. Mirotic won the Spanish League MVP award (which is no joke) at the age of 22, and as an NBA rookie, he was the third best player by Win Shares on a team that won 50 games. He clearly had unique NBA-level talents, and 3 years later, an optimist would say that he's no better than he was when he came into the league. A realist would say he's clearly regressed. While he deserves some of the blame for this, I'm 90% sure that he'd be an all-star right now if he'd been drafted by the Spurs.
Portis has shown less promise but he also clearly has unique talents as a player. He might be one of about 10 big men in the league who can comfortably switch on the PnR, effectively crash the boards and reliably knock down an open 3. He's probably too much of an idiot to ever be a truly good player, but any coach with a brain should've turned him into at least a decent role player.
If either of these players had been developed properly, they would've had the starting role locked up.
3. Inability to assess talent: It took about one half of NBA basketball to see that Markkanen was the most talented player on the roster. And this guy was third on the depth chart? Time and time again, Hoiberg has proven that he can't evaluate his own roster. If he could, we wouldn't have had a pointless position battle.
4. Inability to manage conflicts/personalities: Since Hoiberg got hired, the one constant has been a lack of respect and discipline from his players. Any real coach would never have allowed a culture like this to fester. He pretty clearly encouraged infighting in the Bulls locker room by framing things as a zero-sum game as opposed to a collective environment where everybody had an opportunity to grow as players. The assistant coaches are pretty clearly boobs too and had several opportunities to step in and de-escalate things.
5. A terrible hypocritical culture that prioritize the wrong values: One root cause of this conflict was reportedly the fact that Mirotic preferred to work out on his own whereas Portis was in the Bulls' facilities, something the Bulls strangely prioritzied. This is just a weird priority, particularly given the aforementioned point that the Bulls can't develop talent properly. The three most successful Bulls; players of the last 10 years (Rose, Noah, Butler) all consistently worked out on their own during the off-seasons. The Bulls didn't say anything then. Rewarding performance instead of adherence to arbitrary rules would've prevented this mess.
If the Bulls got any of the above things right, they wouldn't be in this mess. But here we are.