Because the big guys with power are never called terrorists.
If you look at many cases of terrorism, you find that those who are labelled terrorists are the little guys who are not organized governments and who choose to take on a large, organized system. For example, in the troubles in Northern Ireland, the Royal Ulster Constabulary, the official English police force in Northern Ireland, committed all sorts of immoral acts. There are cases where they went into pubs suspected of being hangouts for Irish Republican Brotherhood members and arrested every single person in the pub, including grandmas, young kids who'd been sent to fetch their fathers from the tavern, and people who didn't even live in the area, but happened to stop by for a drink. The people were held on suspicion of being terrorists, and oftentimes, they ended up in jail whether there was good information about them being tied to terrorist activities or not. Some people actually died "accidentally" while in police custody. There are other cases of RUC members who were said to be "off duty" walking into pubs and turning them into shooting galleries, killing almost everyone inside with a barrage of small arms fire. The list goes on and on. But no one ever accused the RUC of being terrrorists, except for maybe the people that actually had to witness those acts, or whose husbands, fathers, brothers and friends were among the dead. That label, "terrorist", was reserved for the IRA, an organization that did not represent a government, but rather a small group of people with no voice. Did the IRA do bad things? Yes, they did. Were they the only ones doing bad things in that whole conflict? No, absolutely not.
I don't condone terrorism, but I understand completely why it happens. It is sometimes the only way for people to get their point across when no one will listen to them and they have no power but the power to fight and resist however they think best. In many places around this world, sovereign governments hold all the power. They use legal methods of terrorism, such as specious arrests and illegal imprisonment to exercise their will on others. The fact that they wear uniforms, have official names, and hold all the power keeps them from being called terrorists.
When a small group of people with no voice and no power have grievances with a large, organized body, they will fight with whatever weapons they have. It may be the pen, but more and more, it is with the sword. The pen may be mightier in the long run, but the results of the sword are immediate, and they get the biggest bang and the most attention. That is the essence of why terrorism occurs, and why some people in a conflict are called terrorist, while others are not.