The article posted explains this pretty clearly. With Rundblad looking like a top 4 defenceman going forward (and a puck mover), the Blackhawks' top 4 looks pretty set for a while with Keith-Seabrook, Hjalmarsson-Rundblad. (Rundblad has been playing pretty excellent lately, take your jaw off the floor). And quite heavy on skilled defencemen. I don't really see a need to shoe-horn Clendening in there. Next year they have a bottom pairing open with 4 young guys fighting for a spot on it before free agency or anything else. Stephen Johns is a big defenceman that has taken great strides this year, and plays a physical brand of hockey and is massive, he brings another element that is missing from the Hawks defence and is almost a lock for the 5th spot. That leaves Pokka, Trevor van Reimsdyk, Tim Erixon and Dahlbeck as options for one spot. They reset the prospect clock on Clendening and don't lose any of their defensive depth.
Now you could argue that they chose to trade Clendening over these other youngsters and therefore they value him less. I would disagree. I would say they moved Clendening because he had value. Let's look at these guys. Pokka was just aquired, developing well and they traded Nick Leddy to get him. They weren't going to trade him realistically. Stephen Johns is a special blend of size and ability, a need for them, they aren't moving him realistically either. Erixon is almost waiver material. Not much value there. van Reimsdyk has been injured most of the year and is unknown, what are they going to get for him, and why waste the potential asset he may become? Dahlbeck doesn't have much value. Would any of these guys bring them an 18 year old with potential to let incubate? Probably not. But Clendening could.
Basically they chose to trade the guy that A) was expendable and B) had some value. That list becomes Clendening in a hurry if you are familiar with their team and a reason Hawks fans talked about dealing him so much. They could have waived one of the other guys and kept Clendening as a 6/7th defenceman and let him work his way up, or they take an asset like Forsling and wish him the best and let some of their other guys hopefully turn into assets. It has nothing to do with Clendening not developing or having majour issues. It's simply running a hockey team. Are there 22 year old defenceman that leap frog entire depth charts of similar players? Sure, but they also don't come along very often and they rarely get traded. The Hawks knew they had to make a move on defence and they chose to use some of their depth to reset a prospect slot and take a chance on developing some value long-term. It's a nice situation for them to be in and the Canucks get a talented young defenceman that is NHL ready to develop. Win-win at this point. Who know what the future holds.