Initially, they didn't want DVDs undercutting theater sales. When DVDs first came out, movies were normally released at many different times throughout the world. Sometimes a DVD would be published in one region before the movie had been shown in theaters in another. As a result, they defined 8 DVD regions.
These days, it makes much less sense. Movies are usually released simultaneously around the world. FWIW (not a lot) there are only 3 Blu-ray regions.
There are also TV marketing issues. Negotiating to get a TV program shown in other countries can take a long time. If people watch it on DVD instead, the potential advertisers wouldn't get as many viewers and less funding would be available.
Sony seems to be one of the more restrictive player manufacturers, perhaps because they also own movie studios.
I'm not saying I agree with any of this, but those are the excuses for why there are Regions.
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