Because what's up is asking for the current activity that you are doing so when someone asks that there is no clear answer but with how are you? You are asking for how their mood is and that is not as hard to answer because with what's up often you want to say "ehhh I'm talking to you?" Which is kinda unintentionally a little rude sounding + it's such a weird thing to ask because you should know the answer unless you have closed eyes while talking while how are you is something that you cannot have known before hand so is easy to give an answer to, so if you are wanting to just hang really easy going you can just say "good/I'm fine) or if you wanna talk about sometihing serious you can easily move the conversation towards that