As someone who has, as you say "open[ed] the door of the closet of religion out into the world of rationalism" (or "reason" as LaszloWalrus would have it) I was a liberal before I even became an atheist, and then before I read Ayn Rand for the first time and discovered Objectivism (which, for clarity, were two separate events). So, I think this is an eventuality that you haven't addressed.
I would suppose (although I only have myself to consider for reference) that this is the case for many budding Objectivists--not so much that their thinking is hijacked or "seduced" by those ever-devious liberals, but that they came into Objectivism with liberal ideas in the first place. These ideas may be remnants of an altruistic religion, or as in my case, something completely separate. They're difficult to shake, essentially because being told that everything you believed prior to discovering Oism was a big, fat lie and you were a damn fool to believe it is not an easy thing to hear.
For my part, and for my atheism, I was never all that big on religion (although I tried to be) in the first place, so discovering the "everything you believed before blah blah blah" wasn't hard to take. However, when this realization is applied to liberalism, which was very ingrained in me and in other people (due to society, as you mention), it is. So the budding Oist is left conflicted, and may reject it in the end, or at least grapples with it (as I do).
Of course, the conflicts and the grappling and general philosophic dithering could be attributed to a weak-willed intellect. So be it. Then, in this case, the rejection of Oism really is "their fault" as stated by Maarten. I think whether or not one can accept Oism fully is contingent upon how satisfied a budding Oist was with liberalism in the first place. If, even before picking up a copy of The Fountainhead, this person could look at some of the policies put in place by our government (welfare, business regulations, etc) and societal norms about morality and say "this is a load of Marxist hippie crap" (in not so many words), then they're more likely to respond to Oism completely. If, however, as I was getting at above, the Oist begins studying Rand, still clinging to their former ideals, then they're more likely to reject it. Also, considering our social climate, societal ideals of what is and isn't virtuous, and what they study in school (if they're a college student), these work to cement those ideals in the budding Oist's mind.
Again, yes, if they really did have any intellectual mettle at all, this wouldn't be an issue. But seriously...a lot of people don't. There are far too many Peter Keatings (or is it Keatingses?) out there.
So...the point is, (finally) that more people do not become Objectivists for the same reason many people do not become atheists--the percieved warm, soft fuzziness of liberalism and religion is more inviting than the percieved cold, hard harshness of Objectivism and atheism. The devil you know, right?
Unfortunately, I apparently posess one of those weak-willed intellects that lacks mettle because I know far too much about this struggle. Sigh. I hope this helps answer (some of) your questions.