Because it will expose their lie that there is a consensus on the issue. Margaret Somerville explains in a Montreal Gazette column:

The likely possibility is, however, that pro-choice advocates and politicians will continue to argue there is no need for a debate.

But if the consensus they claim does exist, they have nothing to fear. And if it does not, then in a democracy a debate is exactly what is required.

Do read the whole thing which busts a pair of abortion myths: the idea that there is a consensus in Canada on the issue and that late-term abortions are rare. On the latter matter, Somerville calls for more and easier access to abortion-related information. Why are abortion supporters not only afraid of debate but scared of facts becoming public?