There are many reasons for this, among which are cultural conditioning, concern about what it might mean to one's own lifestyle, and fear of sticking out like the proverbial sore thumb.
Abortion is just such a mind-boggling evil.
Homosexual advocacy is another. Rampant divorce is a third. The list goes on. As a result, they grasp at any plausible platitude which helps them to feel good about not coming to grips with it. One of the results of this, in my opinion, is the condemnation of one-issue voting.