Arson would have to be one of the hardest types of crimes to trace down because of the nature of the crime. Most, if not al, of the evidence would probably burn up. A good example of this is the wildfires that were started by someone in Colorado, and there is very little hope of finding the criminal that caused all of the damage.
Arsonists are more concerned with observing the end result than the process of it happening, rarely are they there while it is taking place. There are some, at least one in this area, that have been at it for years without getting caught, despite having a pattern. It is not an easy crime to catch.
Arsonists are very tough to catch because the evidence is so difficult to find by the investigators. Arsonist belief they can get away with the crimes because of the hard to find evidence. Arson investigators always have a tough time but they are determined to do their job as best they can.
One thing about arson is that there is usually some connection between the perpetrator and the victim so that the lines can be drawn back to the criminal. Further, witnesses may have seen odd vehicles or people in the area and provide a description. Some other crimes may be much more untraceable or difficult to track.
With today's technological advancements and better crime reporting data, arson should not be one of the hardest crimes to trace. Fire officials are very adept at locating the source of a fire (where it originated), and any accelerants that may have been used to make the fire burn faster or hotter. Many times they can locate a piece of evidence from the fire that can be tracked down to aid in locating the arsonist. I would think people who set off bombs are harder to track down. Less evidence is left behind.