It is important because people have a right to food, clothing and shelter. It all comes down to life. 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 speak up coherently about any issue impacting human life if you are allowing the life itself to become a disposable item.
Abortion is more than abortion.
We speak more often of the preborn precisely because we are trying to undo the unfair distinction made between them and the born. The preborn have equal rights with the born, and we demand that those rights be respected in the same way. Having a universal concern for human rights never excludes a person from having a specific focus on one group of people in need. The preborn, furthermore, are most in need. That was a bad policy.