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Quite simply, because the servers of these websites are located abroad, and the Chinese government cannot directly control them. As a result, if someone posts content on these sites that the Chinese government does not want to see, without the cooperation of these sites, the Chinese government cannot quietly delete them and hold the posters accountable. Note that these sites are not completely free from censorship in various countries, but they only agree to be censored by law. In other words, if someone publishes illegal content on these websites, if the government needs to delete the post, it must have a clear document issued by the court and submit an application before the website can be judged to take measures to delete the post and hand over relevant user information. This is common practice in most countries in the world. But the Chinese government cannot accept such an approach. All websites allowed by the Chinese government to operate in the country must be subject to opaque censorship. People cannot know why each message is censored, and this behavior is intolerable for international companies . Therefore, companies like Google, Facebook, and Twitter cannot enter the Chinese market to operate, and their websites have been blocked for a long time in mainland China and cannot be directly accessed.
As for commercial factors, they are secondary to political factors. There may be cooperation obstruction from some domestic competitors, but this is not the main reason why foreign websites are blocked.