This Pew survey from around the '04 election provides some useful insights into the question of why evangelicals are more pro-life than Catholics. The study breaks down Catholic and evangelical voters into "traditionalist," "centrist," and "modernist" camps: Traditionalists of both religious persuasions are much more likely than modernists to attend church regularly, to have a personal as opposed to impersonal view of God, and to favor "preserving" tradition over "adapting" it, while centrists fall somewhere in between. And this divide spills over predictably into abortion politics, where traditionalists are more pro-life than modernists, and centrists fall into the muddy middle.
But here's the crucial finding: Both in percentage and in absolute terms, there are many more traditionalist evangelicals than there are traditionalist Catholics. Almost half of the evangelical population falls into the traditionalist camp, compared to less than a fourth of the Catholic population; indeed, there are actually more modernist Catholics in the American Church - using the term "in" a bit loosely, I admit - than there are traditionalist Catholics, whereas there roughly four times as many traditionalist evangelicals in the U.S. as there are modernist evangelicals.
These numbers seem to buttress my thesis that evangelicals, as a very general rule, tend to take their religion more seriously and practice it more intensely than do Catholics. But that being said, even when you compare traditionalist Catholics to traditionalist evangelicals, the latter are still slightly more likely to be pro-life - and modernist evangelicals, tellingly, are much more likely than modernist Catholics to support restrictions on abortion. So even allowing for the difference in religious intensity between the two groups, there's room for other explanations as well - like Ed Kilgore's suggestion (which Daniel Larison expands on a bit) that the countercultural aspects of American Evangelicalism tend to amplify skepticism about the current abortion regime in a way that the more assimilationist mentality of post-Vatican II American Catholicism does not. (And see Steven Waldman and Rod Dreher for more thoughts on this subject as well.)