(why does Anti-Semitic mean Anti-Jewish, if Arabs are also Semitic? Or at least, Arabic and Hebrew are both Semitic Languages...)
Originally anti-Semitism did also refer to Arabs, as it was coined in reference to eugenic ideas about the inferiority of the "Semitic races" to the "Aryan race". The key point, though, is that in most of Europe before the World Wars, if you were hating on the Semitic races you were actually hating on Jews because those were the Semites who were available to blame your woes on - Arabs were just too far away to be properly persecuted.
Later, "anti-Semitic measures" were taken to purify the Aryan gene pool. Now, if there's anything that permanently dysphemizes a word, it's being utilized in the context of the highest-profile act of genocide and most destructive war of the modern era. Just as "concentration camp" was intended as a euphemism (see, we're just concentrating their population in one place!), "anti-Semitism" was a term intended to euphemize the killing and sterilization of Jews and so forth. Also like "concentration camp", the result was that the term ended up permanently associated with that use of it - in this case, Jew-hatin'. Or, on the Internet, Jew-h8in'.