1. Expanding cap is one issue. First, more teams will have capspace, so why give up assets to get a guy you could sign. Second, if you have a guy on a long term contract now, he will be "Underpaid" under the new cap so keep him. Likewise, if a guy is NOT on a long term contract, no one wants him because he may bolt and/or become pain under the NEW cap, and not be a bargain any more.
2. The CBA itself discourages trades. First, they have shortened contracts, so there are far fewer burdensome mistakes to dump, and rebuilds can happen much faster. Before, it seemed like people didn't want to acquire a player because they had too many years left; now it seems like no one wants to give assets for a player because they are free agents soon.
3. They got rid of sign and trades and extend-and-trades. This has really dampened down the market; remember the KG deal wouldn't have happened, from either side, without the extension as part of the trade. Now it makes NO SENSE for players to agree to an extension because they can make much more money by becoming a FA and resigning, even if they never intend to leave the team.
4. Because of the above issues, draft picks are more valuable, and if draft picks are hoarded, there are fewer trades. Draft picks are more valuable because you essentially get a minimum 5 years of cheap player, and, realistically, more like 8-9 years of total team control (4 years cost control; 4-5 years 1st post-rookie deal, almost always with the team that drafted them unless the team doesn't want the player).