I'm not sure that's true. The philosophy department at my university has people working on both analytical and continental philosophy. They tell me that most departments specialize in one or the other.

I'm not sure this is true, and if it were, it would likely be violated by the 20 year rule. I certainly learned continental philosophy in college, and it was hardly out of the normal.

However, I would argue that it's the combination of a number of different things. 1, continental philosophy has always been more prone to literary expression than analytic philosophy does. This means that you often find e.g. "The Stranger" or "No Exit" in literature classes, even though the content is distinctly philosophical. 2, there's not a lot of american continental philosophy to build on. Much of the existing corpus of american philosophy is either directly from immigrant philosophers (think: logical positivists) or their academic descendents. 3, much of the recent influential english philosophy that has been produced has been analytic. Bertrand Russel, Whitehead, and Moore were all english, and built on the earlier empirical, english work of Berkeley, Hume, and Locke.

Furthermore, continental philosophy had an immense effect on american academia. The idea of critical theory that forms the basis of today's humanities may be traced directly to derrida, heidegger, de beauvoir, and foucault.

Of course, this isn't airtight; Wittgenstein and Frege were german. Quine, an avid opponent of the Positivists, among others, was american, although I would personally consider him analytic. There's a mix of both memes on both sides of the ocean. I discourage the classification of it by language—I would argue it's more about the approach. Analytical philosophers prefer to use deductive reasoning, either in the symbolic style of the Principia Mathematica or equivalent languages, while Continental philosophers prefer the essay, novel, play, etc., to argue and persuade through non-deductive means, although they may use deduction. It makes far sense to trace a work by its inspiration/origins rather than language; for example, german philosophy would be extremely difficult to categorize between Kant, Nietzche, Kierkegaard, Wittgenstein, Carnap, and Heidegger.