Because while personally some people may agree that abortion is illegal according to the Bible they arent arrogant enough to think its right to force everyone in the nation to conform to our beliefs.
I understand there's no way we can realistically expect an entire nation of people to conform to Christian doctrine. Jesus himself said many will seek to enter heaven, but only a few will be allowed in. As Christians and followers of Jesus, do we not have a responsibility to ask that the tax dollars WE (Christians) give to our government not be used towards abortions? Should we conform to worldly standards because don't want to be perceived as arrogant, and let our government do as it wishes with our dollars? They're doing wrong with OUR money, we have to say something or else we're participating.
The practical side of me knows we can't stop all evils, and that everyone isn't going to be saved, but I still feel like I'm still responsible for standing up and sticking up for what's right or standing up and saying when I disagree with something as a taxpaying citizen, or else I'm inadvertently supporting the thing that's wrong.
They shouldn't expect us to conform to their beliefs.