Because China has the potential to be a superpower while North Korea will never. And also, it wouldn't be a vassal of China; China will defend it without request much less any pledge of subservience simply because China will not tolerate a Korean peninsula unified under American ideology. North Korea can literally give China nothing, ask for nothing, but simply behave in a less aggressive manner building its economy and science normally and quietly and even without any military capability, China will defend it and take care of the rest. North Korea can grow like a larva in a commensal relationship (one in which North Korea benefits while China is simply not harmed or drained) under China's protection and when it becomes big enough, it can then undergo metamorphosis and arm itself with a credible modern force emerging like a giant hornet where it will be able to deal with South Korea from a position of confidence but not necessarily aggression. Instead, it wastes all its energy like some flaccid and gaunt worm squirming around all day hoping that it can train this flailing into a weapon to deter a mantis.
Well you're making a few problematic assumptions here.
First, you're assuming that China wants a strong North Korea on its border. I don't see why that would be the case. Not saying China is doing anything to contain or sabotage NK, but I don't see why China would want to help make NK strong either. Stable? Definitely. A stable NK is very beneficial for China. Propserous? Maybe, maybe not. What does China gain exactly from a prosperous NK? Strong? Definitely not beneficial. A strong NK means it has the option to work with China's enemies to undermine China's security, if it sees advantages to doing so.
Second, you're assuming that China would even want a NK reliant on China for defense. Don't forget that the decision to enter the Korean War was not made lightly, and the logistical challenges of sending troops into NK caused a lot of PVA casualties. It makes far greater strategic sense for China to support a NK that can militarily defend itself rather relying on China's protection. In fact, historically, Korea has served as a buffer zone against Japanese aggression. Yes, China will defend NK if NK is going to get wiped out otherwise, but the idea here is that NK should do everything in its power to ensure that this doesn't happen in the first place.
There's a world of difference between sending in Chinese troops to prevent NK from falling to the US, and sending in Chinese troops every time the US makes an incursion or provocation. A militarily weaker NK means China needs to devote more resources to ensure its security, and I simply don't see China wanting to go that route.
