This is because it is a very subjective concept. This can easily be shown by differing reactions to 9/11. It was condemned by many Western nations as a terrorist outrage, yet in the Middle East and in other parts of the world it was seen as a blow for freedom.
Another problem is what is considered terrorism? It is violent acts from forces outside the United States? Is there domestic terrorism, meaning groups within the U.S. that are undermining the majority culture, such as the KKK? Is terrorism "acts" only, or can verbal threats and hate speech considered terrorism? Because we use the word terrorism rather loosely sometimes, its denotation is less specific and its connotation is expanding as well.
In a pluralistic world definition are hard to establish. Like post 5 states, terrorism to you might be a freedom fighter to another. I think this is going to far, but it does show that there are issues of perspective. There is also the issue of degree. If a country uses the economy to hurt another nation for the sake of personal interest, is this terrorism? Many people get hurt. Some would see this as economic exploitation. Therefore, some might see this as subtle terrorism. In the end, there are many perspectives; this is what makes it hard.
I agree that terrorism has very specific connotations. For example, where do you draw the line between terrorism and rebellion? Would our Founding Fathers have been considered terrorists? They were considered terrorists by the British, and I am sure some other colonists thought so too. As they say, history is written by the winners.
"Terrorism" is pretty much in the eye of the beholder. What seems "terrorism" to one person may strike another person as "fighting for freedom." Violence deliberately aimed at unarmed civilian populations, including children, would probably strike many as terrorism, but, by that standard, many acts of war can be considered acts of terrorism.
Pohnpei makes some great points in his first post. Americans complain about the terrorist activities taking place in other countries, but the USA does not set a very good example for others. Consider the waterboarding techniques implemented with the authorization of President George Bush; or the invasion of Iraq under the guise of there being weapons of mass destruction (with none ever found); or the bombing of schools in the Middle East when it was considered that the collateral damage was acceptable. American politicians and military leaders habitually apologize and claim such things are accidents until their coverups are exposed. It's no wonder that other nations hate America for the double standards we impose.
It boils down, I'm afraid, to how Max Weber described the state, i.e. the organization that "upholds the claim to the monopoly of the legitimate use of physical force." Organizations that cannot plausibly make that claim are considered terrorist organizations, but their victims are no more or less dead than the victims of state-perpetuated violence. There are innumerable gray areas, especially now that states attempt to used professed terrorist organizations to achieve their ends, but the root of the issue is who can use violence, and in what ways.
The above post points to some of the many complexities of clarifying the definition of terrorism. I might add that terrorist groups do not always see themselves as terrorists, but instead can see themselves as freedom fighters, revolutionaries, etc.
There are groups that self-identify as terrorists. These groups utilize a set of tactics that are agreed upon as being "terrorist". Yet, what do we call it when other groups use the same tactics? As pohnpei397 points out, this can be a difficult distinction to make.
Was the Boston Tea Party a terrorist act? No, but how exactly do we draw the line?
This is because one person's terrorist is another person's freedom fighter. It is also because acts that might seem terroristic to us might be difficult to logically distinguish from acts that we approve of or at least tolerate.
For example, people in the American Revolution committed what were certainly terrorist acts. So did the Haganah as they pushed Britain to give them more control over Palestine. But we approve of the outcomes of those acts so we don't see them as terroristic.
Also, thinking about the terror bombing of civilians, how is that logically different than strategic bombing in WWII? The Japanese and German civilians (and the English and French and others) were not combatants. Yes, there was an "official" war, but does that make killing civilians any less wrong.
For these reasons, it's hard to define what is and isn't terrorism.
We are all born as human beings. Then we have been categorised as Christians, Muslims, Judais, Buddhists, Hindus and so on in the name of various religious groups. People have attained their freedom dividing the world in the name various countries, states, provinces etc. Now we are keenly interested in dividing them in the name of community, caste, creed and even cult. Still we are not happy. These days we are busy with terrorising people in the name of Heaven and Hell. Everything are actions and reactions of mind. One mind feels terrorised and the other think of working towards peace. The same act terrorises some and bring happiness to others. People talk of economic growth that will bring to an end or of giving freedom to smaller groups. Can people be happy even if they are allowed to have a own state or to have one religion each to practice? Then we will start proselitizing or imposing on others. What I believe, none can stop these actions unless they whoc work on it. If we try to put of them, that is also an act of terrorism. Therefore, the media should not give much importance to it and should never be published in the mass media. The act of terrorism has increased because we are much concerned about it and give much publicity. Thus they are inspired to act on more.
This is the reason, it is very difficult to define terrorism as by and large we are all enganged with it either this or that way.