Could it be size......they both come in a variety of sizes so no, it can't be that. Could it be how hard it hits, the amount of power it puts out.....they both are capable of putting out a range of wattages, albeit by different means but No, it can't be that. Price......both can range from cheap to insane amounts so it can't be that either. So what does it REALLY come down to?? PERSONAL PREFERENCE, and some people actually prefer BOTH! I also think other factors play a part, such as ease of use, durability, safety etc. There's very good reason why rebuildables come with a CLEAR warning on their use being recommended for ADVANCED users, but an "APV" is called "ADVANCED personal vapor for a reason also!
The answer to your question can be answered with another question. Why are there still traditional pedal powered bicycles when there are electric and motor bikes?? It all comes down to what you need and want it to do, and what you prefer. I personally prefer the personal satisfaction I get from knowing I built something that works, does what I need and want it to do. I like it that I can fix almost anything that can go wrong with one of my mechs with a trip to the local hardware store if need be. I love the looks of the varieties of shiny metals, but the same can be said of DNA devices also. There are high end DNA devices with lots of customization, but I personally don't care for the current limitations of wattage on them. I have both, mostly mechs, but I do have a small DNA35 with a tank on it for those times when it's not convenient to drip lol, but regardless, it all still comes down to personal preference.