Because Ayn Rand was not a philosopher.
When people talk about Rand's "philosophy," they're using that word only as a euphemism. What Rand developed through her books and presented to the world was an ideology, a complex set of beliefs, opinions, metaphysics, and ethics. Philosophy is basically about the appreciation of wise thoughts, not the development of new ones. It isn't simply a set of assertions, no matter how superficially logical.
Internal consistency isn't enough. What's necessary is a rhetorical process that grapples with the philosophical questions of the day and addresses the strengths and weakness of the various solutions. It also requires a literary dialogue with philosophers of the past.
Unlike physics, which exist apart from academia, philosophy is inherently academic. There's a reason that the highest degree granted (in the United States, at least), a PhD, is called a doctorate of philosophy. At a high enough level, all academic work is partially philosophical. When you remove that from the living philosophical tradition stretching back hundreds of years, what do you have? Nothing.
When you dismiss the philosophical establishment altogether to start over with a blank slate, you're not doing it right. Yes, you can say that most of Rand's work was in the field of ethics, but you can say that about Pat Robertson, too. It doesn't make them philosophers. What they have to say is hard to combine with the academic tradition because it's poorly defined relative to it.
Philosophical books, papers, and classes may discuss the contributions of non-philosophers, but they're under no obligation to. Time is limited in every course; space is limited in every book; attentions spans of students are, of course, also limited. Since Rand's ideology is basically self-contained, there's little lost by excluding it from the philosophical curriculum, and little gained by trying to bolt it on.
It might show up in a broad survey course, or on a special class like "Philosophy and Ayn Rand." Otherwise, it won't. And until and unless Objectivists become as prominent in general population as Christians or as prominent in academia as Marxian theorists, I don't see that changing.More questions on Ayn Rand: