Two reasons your cover letter sucks:
1. It doesn't exist. You just send your resume. Adding a three-sentence note in the email doesn't count.
2. It exists, but it might as well not, because it just repeats the same info that's on your resume. Think about this for a minute: Why would an employer want a cover letter that just gives them the same information as you are already sending them in the next document? Answer: They don't.
99% of job applicants, I'm talking to you. Go redo your cover letter and tell me something that isn't in your resume!