Why? I suppose Orwell seems clear and accessible, not so far removed from the way we write now that a student has to engage in quite the amount of work involved in understanding the prose of Austen or Dickens. Those essays are also a good length for a manageable classroom discussion. "Politics" and "Why I Write" are reflexive in their subject matter. The novels are short, and ripe for introducing terms like metaphor, allegory, and irony. 1984 is pleasingly close to science fiction for those so inclined. Then there is ideology: in the American classroom, Orwell often gets painted as nothing more complicated than an anti-Communist—a picture which leaves out a lot, but which will not raise the hackles of parents or school boards. (If the average American better understood Orwell's politics, the chances are good he wouldn't be taught at all.)
What strikes me now is just how much historical context one has to have in order to read Orwell accurately—indeed, sometimes to make sense of him at all. Without that, he easily dissolves into moral platitudes. Just the first two paragraphs of "Shooting an Elephant" require some knowledge of the history of the British Empire at large, its entry into Burma, the place of Buddhism in Burmese life in the early twentieth century, what colonialism is like from the perspective of the colonized, the forces that would draw a young man with an education but no money into being an agent of empire, the leftist anti-imperialism within Britain in the early twentieth century—to say nothing of picking up on Orwell's irony, or knowing the meaning of the Latin phrase "in saecula saeculorum." Offering all this to ninth graders, or even college freshmen, with almost no background in the relevant history is a herculean task for even a very gifted teacher. My experience has been that classroom use of this essay tends to focus on form and rhetoric, which serves as a convenient way to elide all the context except as needed. But a young reader—indeed, even many an old reader—is left clutching at the ghost of what Orwell means.
And these essays and novels are even cherry-picked for being relatively light on history. I suspect that at least half of the reason why essays like "The Lion and the Unicorn," "Looking Back on the Spanish War," and "England Your England" aren't taught, brilliant pieces of writing though they are, is because they require too much historical and cultural knowledge—of Britain, of the interwar and War from the British perspective, of a political vocabulary that can't be easily imparted to students in five to ten minutes. The other half is Orwell's socialism, which demands a distinction between the democratic socialism Orwell espoused during the period of his best-known essays and the totalitarian Communism he abhorred at the same time. Americans, at least since the McCarthy era, have never been good at making this distinction due to our anathematization of socialism as tantamount to Communism. Maybe the current resurrection of socialism in our twenty-first-century political vocabulary—at least in the form of Senator Sanders in the last election—may change that; maybe not. But I suspect that many an American principal (or dean) still can't countenance the idea of bluntly acknowledging that a man lionized in popular culture and school curricula for his anti-authoritarianism was a democratic socialist.
Use and abuse of language, and what it looks like now, is the other vicissitude of the passage of time on understanding Orwell. Often—too often, I think—"Politics and the English Language" is presented as a perfectly valid style guide to young writers. It rails against overused metaphors, highfalutin polysyllables, blandishing understatement, insidious euphemism—all things that teachers of writing (rhetoricians and grammatici, if you will) try to knock out of their students. But those are not all the rhetorical crimes of 2017. Orwell was writing in a time when a veneer of sophistication (emphasis on "sophist") could still be used to communicate authority—in Parliament, in a newspaper, in a book. Read enough prose of the British 1920s and 1930s, and you come to understand: everyone is trying to sound like they have a classical education. That still happens from time to time in the academy or in corporate reports or the like.
But we live now in an age where the more widespread, public danger is fake simplicity, fake bluntness, fake folksiness. No American politician uses big words to mask their meaning now: they use small ones instead. If anything, the American public has absorbed Orwell's suspicion of big words too well, and now takes simplicity as a sign of truth and authenticity. The lesson we need now, in the age of the YouTube commenter, is that little words lie just as well as big ones—the fear of the deliberate impoverishment of language illustrated by the Newspeak of 1984. But too often we don't teach our students that what Orwell was really getting at was not that words, the big and the small, are somehow both bad, and we should aim at some golden mean consisting of words between two and four syllables of five and ten letters. He was trying to show us what it sounds like when someone is trying to hide the truth, and teach us not to hide it ourselves. Any formulae Orwell gave were just guidelines toward that goal, not rhetorical dogmas he thought we should worship. Teaching this skill—clear, true thinking—is still hard for even the best of teachers. At the risk of sinking into platitudes, you could say it has motivated every teacher of rhetoric since the time of Plato. It is hard to not to worry that we are losing the battle.