What the phrase means isn't exactly clear. Other issues exist because people exist. If there were no people, there would be no issues and nobody to discuss them. The bottom line, in other words, is life.
Any issue is important because life is important.
It is a concern because people have a right to make a living. It is important because people have a right to food, clothing and shelter. That's why abortion is the key issue. Deny that a person has the right to live, and you undercut the importance of every other issue. It is impossible to coherently speak up about any issue impacting human life if you are allowing the life itself to become a disposable item.