Because we know kids can get access to vaping, just as almost every (ex)smoker reading this was able to get access to cigarettes when they were not legally allowed to buy them, it is PRECISELY why we SHOULD be allowing legal access to them. The alternative is for kids to fend for themselves, be put in position of buying from underground (arguably more dangerous) market, and then learn for themselves why the product was denied to them in the first place. Then, find out the truth, and only be able to convey pros/cons for use amongst themselves, with the spin that only minors can put on things, and coupled with stage of human development where peer pressure is arguably at its peak.
If we allow legal access, there is better chance that minors, on the whole, would get more balanced perspective about what it actually means to use nicotine, as a recreational drug, from older folks. Both in terms of what it is truly like to use one or two (aka in the short term) and what it is like to use habitually over the long term.
By denying legal access, it means adults are kept out of the discussion regarding teen use, and to the degree that is not true, then it very likely means that all that kids will hear from adults is "you shouldn't do this, because you're not old enough" and if that doesn't work, then let us tell you all that is wrong with it, and let you discover on your own, or from peers that have tried it, all that might be good. Once you learn that we adults were lying to you about the good, or refusing to discuss that with you, we won't be surprised if you essentially don't trust us on other matters, because in shared reality, we were no different than you when we were that age. Some of us clearly knew that smoking was a way to act out / rebel against establishment (read as adultism) which was another benefit for using, even if adults may vehemently disagree with that sort of rhetoric.