Why would I be unable to find certain seasons of TV shows on DVD?

I don't understand this. For example there is a show on National Geographic called Drugs INC. You can buy the DVD of season 2 and season 3 but not season 1 for some reason. I was also trying to buy a show called ghost adventures which goes up to 11 seasons but I'm only finding up to season 5 on DVD.... show more I don't understand this. For example there is a show on National Geographic called Drugs INC. You can buy the DVD of season 2 and season 3 but not season 1 for some reason. I was also trying to buy a show called ghost adventures which goes up to 11 seasons but I'm only finding up to season 5 on DVD. I'm starting to get frustrated and I don't understand why this would be. Why can I find certain seasons only available for purchase? I've looked everywhere. Does anyone have any idea why this would be?
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