Why have Americans gotten more liberal on most social issues, but are largely unchanged on abortion rights?

Over the past several decades, Americans have become more progressive on a host of social issues at an incredible pace: they are far more accepting of gay marriage, supportive of marijuana legalization, and opposed to the death penalty -- to name a few.

One major issue stands out as a notable exception, however: abortion. Since the 1970s, American's attitudes on abortion rights have remained largely unchanged. Pretty consistently, about half of Americans think abortion should be legal under some circumstances, ~25% think it should be legal in all circumstances, and 15-20% think it should never be legal (I presume the remainder are unsure. If you look at the link, the stability in these attitudes is remarkable.

Why have Americans' attitudes on abortion remained largely unchanged while they've shifted tremendously on other social issues?