Because torture is done for two main reasons, both of which are morally indefensible. 1) To gain information, except it has been quite conclusively proven that information gained under torture is unreliable, and 2) merely to torture and intimidate minorities or opponents, which is clearly morally wrong and reprehensible, a crime against humanity.
We cannot claim to be a democracy or a civilized society and at the same time endorse a policy of torture.
Read the Constitution. Find the phrase "Cruel and Unusual." The founders put that in for a reason. Those who condone or practice torture, whether here or abroad, for whatever reason, not only are acting unconstitutionally, but undermine the moral foundations upon which this country was founded. Hypocrites!
The supposition in the question is in itself flawed. Torture does not protect national security. As the previous post explained, torture does not guarantee reliable information at all. As more and more information has been forthcoming about the "enhanced interrogations" conducted after 9/11, the facts support this. Those subjected to torture will say whatever they believe their interrogators want to hear--or anything at all--to stop the pain.
I recently listened to a former military interrogator who was trained in traditional methods of interrogation explain that torture is effective only when those administering it are seeking not information, but a desired confession. This was the intent, for instance, when John McCain was tortured in North Vietnam and finally "confessed" to war crimes and praised his captors. Torture can be used to break someone, but it is ineffective in obtaining useful, trustworthy intelligence. It is not widely recognized, but the Bush administration stopped using waterboarding during the President's first term because it did not achieve the desired results.
There will be many answers to this, so be prepared to sift through what you consider to be good, true, and valuable. The first reason why torture should not be used to justify national security would be that it lessens a nature's moral stature. When a nation presumes to stand against lawlessness, disorder, and barbaric cruelty, it loses credibility when it tortures for it does the same thing it criticizes in others. Whether one calls it "enhanced interrogation" or not, actions such as water boarding or sleep deprivation or humiliating a suspect in a demonstrative manner reduces the moral stature of the nation engaging in such actions. In this light and from a moral standpoint, there is little difference in behavior between statesmen and barbarians. This actually increases resentment, as it creates a rallying point for other individuals who see their brethren degraded on a large scale at the hands of a specific nation. Another reason why torture cannot be considered reliable is that the information it would procure is not reliable. If someone is tortured for valuable information, they would render something, anything, in order to stop the tortuous behavior. This information is given not out of a way to stop actions against the state, but to stop the torture. Just because a suspect gives information under duress does not make that information reliable, as it is difficult to ascertain their motivation for giving the information. Finally, torture as a method of gaining intelligence degrades the actual and legitimate intelligence work of men and women in the field. When a nation engages in torture, it is telling, in effect, to its intelligence workers that their efforts are not sufficient. It is saying to them that since information must be procured through torturing, the information gathered by legitimate and respectable means little or is worthless.
If a war, that can kill thousands or even millions of people, can be justified to protect national interest, I see no moral justification in opposing torture to to protect national interest.
I would like to make it clear, that I am not supporting either war or torture as such. I am against both. But if a war is thrust upon a country, war may be the best way to respond. And if torture can help to avoid such a war, or to cut short the losses and horrors of war, it is justified.
Previous posts have argued against torture, among other things, on the grounds that it is not effective. This contention is not supported by reality. Torture may be ineffective in some cases, but it definitely effective enough in many other cases. Some operations performed by doctors fail, this is no justification for avoiding all medical treatments requiring operation.