We who are pro-life do not say that abortion is the only issue. It is, however, the foundational issue. Many things destroy human life. Yet abortion goes beyond that. Our nation's current abortion policy authorizes such destruction by a direct denial of the protections granted to persons under the U.S. Constitution.
An example will clarify this. Sometimes people are heard to say that poverty is a more important problem than abortion. Certainly, the problems are related, and we must actively care for the poor.
But to make a truly equivalent parallel between the plight of the poor and that of the unborn, one would have to imagine a policy whereby a) the poor were officially declared to be devoid of "personhood" under the Constitution (as Roe vs. Wade did to the unborn), and b) over 4,000 of the poor were put to death daily against their will, while efforts to directly save them were prosecuted by the government (as is the case regarding the unborn).