It's coaching. The Bears always act like draft picks come fully formed and can't get any better. So they never try to coach up players (and often coach them down by forcing them to do things they aren't good at).
The trick is to get a good draft pick (and to be fair, many draft picks are good enough to compete with the proper coaching and support). Then you properly coach them and build a game plan around their strengths (rather than forcing them into a system where they play poorly). That gives you a chance to win.
The Bears consistently do the opposite. Draft a player, ignore his strengths, force him to play a system that he does poorly in and give zero coaching to make them better. And strangely, it's a disaster (well actually, it often is at just mediocre, which is worse because coaches don't get fired for being mediocre in Chicago, and you end up with middle tier picks).
I don't see this changing until the ownership decides that winning is important. And I can't see that happening as long as merchandise and tickets sell.