The devices themselves have varying liquid capacity and vaporizing volume settings--you can't easily track how many pulls until the device is empty.
Battery limitations also confuse the amount "left" in the device.
Cigarette units are simpler, in part because there aren't that many brands, and even if you roll your own, you probably have a pretty good idea of how much tobacco you're putting in the cigarettes.
But especially with boxes of cigarettes, you can easily count them down and keep track knowing each starts with 20.
I'd also suggest that cigarette use is far more difficult to hide than vaporizer use. Cigarettes smell terrible compared to most vape clouds. The smell and risk of iscovery could be important elements of cigarettes that might make them easier for teenagers to quit.
Finally, vaping comes in just about every flavor imaginable. Much more diverse in taste offering than cigarettes with the same high. It is really a much more refined narcotic delivery than cigarettes and so it aught to be harder for anyone to quit.