I don't think everybody believes it is a cowards way out. In some situations, people truly are being cowards for committing suicide. A lot of people make a big mistake in life, and instead of taking responsibility, they run away by CTB. Aside from people who use suicide to run from a mistake of theirs, I think the other reason people may view suicide as cowardly is because it is often a permanent solution to a temporary problem which affects everyone in that persons life. A lot of young people who CTB do it irrationally because they can't grasp how short-term their problem may be. I knew a teenager who CTB cuz he broke up with his gf. Now that may sound like a silly reason for a teen to CTB, but that teen thought it was reasonable; he couldn't imagine spending the rest of his life without his gf.
Those are some reasons people say suicide is cowardly. But not everybody thinks it is, and not everybody CTBs for the same reason.