Merely comes from the Latin merus, ("undiluted"). But "undiluted" is positive — as in, "that's the undiluted truth" — in a way that merely is not. When you use merely, it always has a negative, disapproving tinge to it: You might say, "She was merely a minor player in that drama," but if you said, "She was merely the star of the show," you would mean it ironically, since being the star is the most important role.