Becuase the Bear's decided to play games in the interview process and interview everyone and their mom. Ben could have taken it as they don't value him as a candidate and appreciate what he brings to the table as an offensive playcaller.
This goes in line with organizational alignment. You want everyone bought into your vision. If not, you may have to fight to get that high cost free agent, or draft prospect, or to receive the support you need to carry out that vision.
Meanwhile, a job like Steelers open up while you are being indecisive and the Bears franchise no longer has leverage. It lost its advantageous position. The Steelers can offer long term stability. Their fannase is more content than ours. Its a better environmebt, etc. There could be multiple reasons.