I'd say a lot of evangelicals and protestants are as vocally opposed to abortion as Catholics are. The problem is that the Roman Catholic Church is the ancient church of the apostles, and Church teaching isn't just seen as a conclusion drawn from what the Bible says, but as the inerrant and unchanging Word of God. [1] On the other hand, all protestant churches - to my knowledge - reject this. On the other hand, they believe in self interpretation of scripture and many of them harbour "liberal" attitudes towards social issues such as LGBT issues, feminism, and abortion.

The Catholic Church teaches authoritatively that procuring an abortion, performing an abortion, or assisting in abortion is a sin of grave matter and doing so incurs excommunication latae sententiae - automatic excommunication from the Church (Canon 1938). The family is at the heart of Catholic social teaching (henceforth referred to as CST), and abortion is viewed by the Church as being an assault on the natural family.

Most importantly I think for America today is the role that Pope Saint John Paul II played in mobilizing Catholics against abortion. In 1981, he was viewed favourably by 93% of Americans (Gallup). I don't know the figures among Catholics, but I would confidently guess it was even closer to 100%. St. JPII authored the encyclical "Evangelium Vitae", meaning "Gospel of Life", which explicitly condemned abortion. Here are some quotations from it:

"Whatever is opposed to life itself, such as any type of murder, genocide, abortion, euthanasia ... all these things and others like them are infamies indeed."
"Among all the crimes which can be committed against life, procured abortion has characteristics making it particularly serious and deplorable."
"I declare that direct abortion, that is, abortion willed as an end or as a means, always constitutes a grave moral disorder, since it is the deliberate killing of an innocent human being. This doctrine is based upon the natural law and upon the written Word of God, is transmitted by the Church's Tradition and taught by the ordinary and universal Magisterium." [2]

The reason why Catholics are so vocal about this however? Participation in the political process is also an integral part of CST. The US Conference of Catholic Bishops says quite simply: that "participation in the political process is a moral obligation." [3]

Sources
[1] Catechism on the Magisterium
[2] Evangelium Vitae
[3] Minnesota Catholic Conference