1. It never occurred to them that anyone other than humans would subject to death because of Adam's sin. The works I'm referring to don't spend any time arguing the point, they just assume it and assume that their audience will do so as well unproblematically.
2. The influence of Aristotle. Aristotle taught that all earthly bodies were inherently corruptible. Hence, death and corruption were nature to animal bodies.
3. The natural instability of created being. Athanasius remark in the On the Incarnation of the Word, that humans and all other created beings are inherently unstable due to the fact that they come from nothingness. Therefore they have just as much a possibility of returning to nothingness. Therefore, he argues, had Adam not fallen, the would have bee deified to preserve his immortality, but he would not naturally have been immortal.
George also brought up Augustine and therefore raised the point of how creationism was understood prior to the 20th century. A couple of points about this.
1. Augustine thinks the days of Genesis are figurative because he wanted to solved an exegetical problem. Eccleasticus in the apocrypha says "Oh Lord, you have made all things at once." But if that's the case, Augustine said, how is it that Genesis says seven days? His solution was that the days were figurative and represented a period in which "seeds" of life which God had planted all at once, had unfolded. In other words, God did make all things from the seeds "all at once" and then figurative seven days were just an unfolding of them. So, when modern theologians want to use this to justify allergorizing Genesis 1, it doesn't work, because Augustine's problem isn't the desire to reconcile science with religion, it's to solve an exegetical problem (one which Protestants don't have, being that we don't have the Apocrypha!).
2. It should be noted that prior to the mid-20th century, most conservative Protestants in America and Britain were either evolutionary theists or old earth creationists. B. B. Warfield for example supported theistic evolution. Most of the people who wrote the "Fundamentals" which the word fundamentalism is based on, were old earth creationists. In Britain, C.S. Lewis was a theistic evolutionist as well.
3. What happened? Specifically the change is due to the Seventh Day Adventists and the Lutheran Church-Missouri Synod (two groups that have almost nothing in common with one another). The Seventh Day Adventists were committed to young earth creationism for two main reasons: 1. They believed in literal obedience to the Sabbath (which wouldn't work with a figurative seventh day of creation!). 2. Ellen White, their founder, claimed to be a prophet. In one of her visions, God had taken her back in time to see each day of creation. Hence, since she saw seven literal days and because they didn't want to admit that she was a false prophet, they were very insistent on young earth creationism. Therefore Adventist scientists set about inventing modern creation science, along with flood geology and what not.
Enter the LCMS. The LCMS was also committed to young earth creationism because of Luther's commitment to it in the Genesis commentary (Luther was actually the first exegete since about Irenaeus to take the account completely literally). As we are all well aware, the LCMS has an extensive school system. Therefore, a group of LCMS science teachers developed materials for use in Christian schools promoting the creation science developed by the Seventh Day Adventists, which was eventually picked up by other Protestant denominations.
Of course, Conservative Christians gradually became more and more suspicious of theistic evolution and old-earth creationism because of its associations with secularism. There remains a conflict of world views (between theism and evolutionism) to say the least. What many seems unaware of is that it all goes back to the debate in Antiquity between various forms of theism and Epicureanism. I find it quite annoying when it is discussed as a question of "science vs. religion" (as the media does frequently) because really it's a philosophical question between two types of causality and therefore two different scientific paradigms. Commitment to either is rooted in a prior worldview- though I would argue that in analyzing the different world views, Theist creationism is inherently more rational.