Let me start by saying that I am fully aware that Roquan Smith is an incredible player and he gave his heart and soul to the Bears' franchise. Smith was the pillar of the defense for years. However, it looked like he was not the best fit inside Matt Eberflus' defense and the Bears may a did a great thing by getting a 2nd round draft choice in exchange. In addition to that, it was pretty obvious at this point that Smith was not going to re-sign with Chicago.
here is a video where you can see a few things that made Smith a no-so-perfect fit in Chicago.