Human Beings (as much as we might not like to admit it) really are creatures of habit. As a result, what we do on a regular basis speaks volumes about how we will behave in the near future. That's why banks want to view your credit history (your buying and paying habits) before giving you a loan. That's why schools want to check your high school grades before accepting you into college. And it's also very much why employers developed the "past performance predicts future behavior" mentality which requires job seekers (you) to provide resumes and job histories in order for employers (them) to predict how you will handle the job they currently have open based on how you've handled your jobs in the past.
Unfair you cry! Can't people change, you ask? Absolutely. But in most cases, it's only when something truly significant happens in our lives that we can force ourselves to the hard work of dropping our inefficient old habits in favor of acquiring more effective new ones: getting married (or even getting divorced) can spur us to lose weight, battling a serious illness encourages us to exercise more and reduce stress in our lives, while having a child can be a tremendous incentive to grow up
As a result, unless and until a person has a "Road to Damascus" experience (getting knocked on your butt - hard enough to change your world view), it's assumed we will continue to do what we've done before. So if on further review your own job history has blemishes, an employer will be a lot more open to giving you a "second chance" if you're able to tell them what happened to you on your own "Road to Damacus" that caused (or is causing) you to rethink how you've done your job in the past in favor of doing it better in the future.