The term "Creationism" describes one
theist position on the history of Earth. It holds that Earth is about 6000
(maybe 8000) years old, and that the "God" of the Judeo-Christian Bible created
it out of nothingness, exactly as that series of events is described in the
Book of Genesis. In this regard there are folks, for example, who regard as
factual the presentations at the Creation Museum in Petersburg, KY, where, to
quote from their website, "Adam and Eve live in the Garden of Eden, children
play and dinosaurs roam near Eden's Rivers [and t]he serpent coils cunningly in
the Tree of the Knowledge of Good and Evil." To account for certain of the
findings of what others regard as the science of palaeontology, at the museum
it is, for example, postulated that early men rode around on dinosaurs.
Why no bridles have shown up with Tyrannosaur or velociraptor fossils is apparently a question that has neither been raised nor answered. Maybe early man just guided these small-brained reptile predators with their hands. But that's the kind of question a scientist might ask. Forgetting about science or science-not, that there are in other cultures around the world many other creation stories involving one or another forms of the "divine," with no indication from any source about which, if any, inerrant or otherwise, might be the accurate one (if indeed any of them are accurate and how one would prove that in any case) is a matter that does not seem to concern Christian creationists.
Creationists believe what they believe because they believe that the whole of that book called The Bible contains the "inerrant word of God." This means, to my understanding, that a) "God" wrote it, or at least dictated it in one sense or another, and b) that every description, proscription and prescription in it is his, her, or its word and is to be understood literally and followed to the letter. Although I don't know of any polling on the matter, my impression is that the text to which most U.S. creationists refer as "inerrant" is the one found in what is known as the "King James Bible." Which before we get to the question "why creationism" does raise some interesting questions of its own.
The "King James Bible" is a version of the ancient text that over time was written/translated in/to/from Ancient Hebrew, Aramaic, Early and Classical Greek, and Latin into a wide variety of modern languages. The King James version was created in England at the time of the accession of King James VI of Scotland to the throne of England, as James I, in 1603, upon the death of Elizabeth I. While his mother, the tragic figure Mary Queen of Scots, was a Catholic, succeeding to the throne at a very early age upon Mary's abdication, James was raised as a Protestant (and was, by the way, according to some sources gay. If true, that's quite a contradiction for the "the Bible-is-inerrant" Christian homophobes, is it not?)
Having experienced religious wars, on and
off, through the 16th century and having seen the effects of much
more vicious ones waged during the same period on the Continent, the English
ruling class had no interest in possibly returning to the days of armed
religious conflict. And thus there was a major interest in establishing a
firmly Protestant, English, Bible as "The Book." And so "The King James
Version." This translation and compilation, from multiple sources in several
languages, by a committee of 47 Anglican Church scholars, was published in
1611, with a further-edited version published in 1769.
Now it happens that there is no evidence that any of the Committee's members had any direct communication with "God." Thus there is no evidence that the King James Version, is in any way the "Word of God," inerrant or otherwise. However, this circumstance, like others mentioned above, has never seemed to have stopped any believer in "Inerantism" from believing that every word of this particular multi-sourced/multi-translator translation is indeed the "literal Word of "God'." So why is this a problem in the US? If folks want to believe in an "inerrant" Bible, take the descriptions of events literally and follow its prescriptions and proscriptions as they wish, what's the problem with that?
It's a problem because there are "Inerantists," generally found in the Religious Right who not only want to take various of the Bible's prescriptions and proscriptions as guides for their own behavior, but they want to force them upon the rest of us. On certain issues, like the matter of when life begins, they want to do this through the use of the criminal law. On others, like the matter of gay marriage, they want to use the civil law to enforce their religious position. These policies can be best characterized as ones of "Religious Determinism." Of course the Christian Right and their Republican allies, often indistinguishable from one another, as when Billy Graham openly endorses Mitt (whatever happened to Mormonism-as-a-cult) Romney for the Presidency, pick and choose very carefully from among the Biblical prescriptions and proscriptions that they want followed. For example, they forget about "Thou shalt not kill" and totally ignore the Dietary Laws. But that ultra-selective (actually politically-driven selectiveness) is a matter for another time.
The point here is that if those rules found in the King James Bible that the Republican Religious Right likes and wants to impose upon the rest of us through the use of the criminal and civil law, are justified by them based on the supposition that they are the "inerrant word of God," then the whole of the Bible would have to be "inerrant," not just the selected rules. And that is the "why" of creationism. For if it just a story, created by humans, not "God," then the whole claim of "Inerantism" for the rest of the Bible falls flat on its face. And so then would their entire justification for the use of Religious Determinism in national policy. Presently, Republican Religious Determinists fall back on selectively quoting the "inerrant Bible," from the Halls of Congress on down (or up, depending upon your view of the Republican Congress) to justify their position. If they didn't have that crutch, why then they might have to argue from a position of reason. And good golly, where would that leave them?