While feminists see abortion as a feminist issue (and I agree with that), it can also be argued that it's a moral/ethical issue. And as such, the broader community has the right to say if something is acceptable or not.
Less of a legal issue, but it wasn't that long ago that men and women living together without being married was seen as scandalous. Slowly, society as a whole changed its views and while there are surely many people who still think it's wrong, the overall view of our society is that it's okay. That sex before marriage, is okay etc.
I think abortion is complicated because it does come into many different areas of people's lives. As a woman, it's clearly about bodily autonomy and the right to control what we do with our own bodies. But for others in our society it as much a religious or ethical issue. And interestingly, I've read a few blogs and articles over the years from traditionally religious feminist women who struggle with this exact concept. They have sympathy for the bodily autonomy argument, but they have ethical and moral concerns about abortion. For them, it's a real dilemma.
However, I think that when your'e changing your constitution you do need everyone to be able to vote. And yes, that puts abortion more firmly into the ethical/ moral category rather than the feminist category, which feels wrong for me. But overall, I think it's best.
Having said that, abortion being wrong in the constitution in the first place? Make it a law, sure. But blimey, we wouldn't be here in the first place if it hadn't gone there.