Answer: Objectivism is true, and it does make sense. Because of this, it is easy to understand and to live by. But it also stands against many of the received traditions in philosophy and religion. And because of this, most people find it counter-intuitive and hard to integrate into their existing values and emotional commitments.
All people have what Ayn Rand called a "sense of life." A sense of life is an emotional sum, a feeling of how the world is and how it ought to be, that serves as the equivalent of a kind of basic philosophy. If you thrill at the idea of taking responsibility for your life's major problems, or you shrink at it, or you feel wry resignation, or you feel something else, whatever you feel in facing such a big question reflects your sense of life. Like the values that drive all our emotions, we hold our sense of life subconsciously, and it is not always easy to identify its content or judge whether that content really makes sense.
Explicit philosophy is a system of abstract ideas. Unlike the value-judgments behind our emotions, abstract ideas must be held in conscious focus to be known. Our consciousness is a volitional process of awareness of the world. But because it is volitional, we have a continual challenge to keep our abstract concepts and theories grounded in reality. Most people let their abstractions float disconnected from facts (just look how words like "freedom" and "justice" are tossed around), and they don't always do a good job of connecting their practical, concrete actions up with their theories.
Most people's senses of life, as well as their explicit philosophical commitments, have been heavily influenced by traditional philosophy and religion.
Many of these traditional ideas embrace Intrinsicism, which is the tendency to think that the contents of the human mind are simply embodied in the world (the idea of a God who personifies moral law is an example of Intrinsicism; so is G.E. Moore's idea that Goodness is a property embodied in things as such, like physical mass is).
Many others embrace Subjectivism, which is the tendency to think that the human mind creates its own contents unconstrained by reality (Buddhism is an example of this in religion, because it holds that we can literally escape from ordinary reality through disciplining our minds; an example from philosophy is Immanuel Kant's idea that time and space are creations of the human mind).
Most traditional views of ethics and politics view goodness as based in self-sacrifice and service to others, and see the good government as one that controls and orders society for the best in all details (this is as true of conservatism today as it is of socialism and communism). Furthermore, most people get their philosophical ideas from religion, and religion is necessarily committed, at least in practice, to unreason, because religions appeal to the idea of a supernatural reality beyond causality and beyond the ken of the human mind, which is ultimately known not by reason but by faith.
Asking people who have been brought up in these traditions (and who are not already prepared to leave) to abandon their ethical, political, and religious commitments naturally clashes with their emotions and sense of life. Many find Objectivism's atheism hard to swallow—they are deeply attached to the fantasy of a personal God watching over them and giving life meaning. Many find Objectivism's ideal of rational selfishness hard to swallow—they've been taught all their lives through every movie and every TV show and every novel that heroes sacrifice themselves for others and that it is noble to have pity for the poor and suffering. Many find Objectivism's politics of classical liberalism hard to swallow—they've been taught that economic liberalism is immoral or inefficient, that it caused the Great Depression, that it would create monopolies, that it wouldn't work. And in any case, people like to preach policies in politics that they wouldn't choose themselves, just to talk virtuously. Finally, most people do not think independently in the first place, and would find it painful to buck against the assumptions of their family, tribe, party, and/or religion.
Against thousands of years of these kinds of traditions, and against widespread hypocrisy and anti-conceptual attitudes, Objectivism has had in its favor: 1) its truth; 2) its close relation to Aristotelianism, Thomism, Lockeanism, and the outlook of the Founders of the United States; and 3) the popularity of Ayn Rand's fiction. Of these, only Ayn Rand's fiction has really served as a powerful marketing force for the ideas, and what a marketing force it has been! (Like many, I first encountered Objectivism through Rand's fiction.) However, like any art, its appeal is limited by its medium and by the fact that it concretizes Rand's own particular sense of life, which will not and should not appeal to everyone.
To spread farther, Objectivism will need advocates and development on many fronts. There is a crying need for new artworks that concretize its ideas, showing their meaning. The ideas need scholarly development, to demonstrate their power to the intellectuals in technical philosophy and related fields. They need practical development: Objectivists need to be able to appeal to a rich body of "technology for living" to show people that Objectivism works and how it works at a personal level. And Objectivist ideas need steady and powerful advocacy in the culture at large, to keep reminding people who have been inspired by Rand's ideas what they mean and imply, and to introduce more and more people to the key insights of Objectivism.