"This has been attributed to more sophisticated, more powerful IEDs that are being deterred more effectively."
More like used more sparingly. They are more effective individually, being harder to detect and resistant (if not immune) to ECM, are more often directional to defeat armored Humvees and armored private security vehicles. But at a cost of being harder and slower to make. I've seen some impressive performance by some of the latest armored sedans & SUVs against simple blast-IEDs. One good example is the VBIED that was used in an attempt to assassinate an Iraqi general, all caught on video. There was not much more than a car-length or two between the VBIED and the target in front and the security SUV about the same distance behind when it went up in a massive blast. Only the bomber was killed.