Why are only Muslims considered terrorists?

They are misinformed and probably do not personally know any Muslims.

It comes from an unfortunate tendency for human being to generalize from small samples. Thus, the only "Muslims" some people know of are of the 9/11 terrorists. As these are the only Muslims known, people then assume that all Muslims are like that.

This is an example of stereotyping. Stereotyping, in its extreme, can lead to racism, classism, gender bias, and a whole host of other problematic behaviors.

From a anthropological/behavioral science standpoint, stereotyping is an evolutionary survival mechanism; it allows a person to make some base assumptions about how to react to a situation based on prior experience. However, it needs to be countered by willingness to expand experiencial knowledge based on new facts.
Some people are so gullible to believe anything the media says and sometimes the media lies that's why