Let me ask you a question: Why should it be illegal?
My guess (and feel free to correct me if I am wrong) is that you will say that you disagree with it, believe it to be murder or perhaps think it is morally wrong. But any one of these things is an opinion.
I'm not going to say if you are wrong or right, because everyone is entitled to their own opinion.
The issue around the legalisation of abortion is that some people disagree with it and some people agree. So by making it illegal you would anger the people who agree with it, but by making it legal you would anger the people who disagree with it.
Personally, I think the solution is to make it legal. People who disagree with abortion don't have to have one, but at least the option is there for those who agree with it.
The problem, it seems, comes when those who disagree want to stop those who agree from doing it.