They make typos and other sorts of mistakes. They don't go through the proper due diligence that you would if you were going to send a package via snail mail. I'm qualified for but not getting much response. A lot of people don't consider the browser or the format that the recipient might be using. As I mentioned, pay close attention to detail.
Many people make typos in cover letters.
List the exact title of the position in the subject line, and your name. Managers receive hundreds of e-mails, and just as you want to be able to search easily, they want to be able to search applicants and the positions they want easily. I have had prospective employers request resumes in plain HTML format. And don't put in any plug-ins or heavy graphics that will possibly slow down your prospective employer's ability to review it. Just the facts.