Free agency is usually a better option. We have had a roster that has proven to be back to back Stanley Cup contenders so there hasn't been a need to drastically change the teams make-up(which very well can change post-Lidstrom), and we have had little cap space, which makes it tough to trade roster players for roster players.
Another reason is the cost of a lot of these players, I was really high on the wings going for Brent Burnsa while ago, but San Jose really really overpaid for him, and made their team worse in my opinion. San Jose had to give up a pretty good second line player on a great contract(setoguchi), their best prospect at the time(Coyle), and a first round pick. We would have given up Filpulla, Smith/Nyquist, and a first for Brent Burns, or maybe even more considering Filpulla's contract isn't as valuable as Setoguchi's. Personally I'm glad we passed on Burns, and tried our luck going after Suter. That's just one example, but other than Burns there hasn't been that many #1, or #2 defenseman traded the past few seasons, especially ones that would fit with Kronwall's style.