I was hoping for a more insightful answer than: because people like to get high.
It might very well be "to get high" ;)
A value in re-asserting a broad, cross-cultural interest in getting high is that it helps demystify it, helps people realize that the drive to get high it is not because of week will or moral turpitude or twisted genes.
It, like the desire to have kids, is simple a common, basic, human desire.
The example of LSD's fall from popularity is correct as far as it goes, but the LSD market was so distorted that it's a poor example. The DEA couldn't get a handle on it when it was hot (I recall reading a policymaker summary complaining that LSD dealers seemed largely unmotivated by money and were thus hard to track by normal means) and a single supplier who produced `90% of global supply isn't exactly normal either.
I do agree with the underlying premise of the article that a well-informed public is likely to produce more users but fewer casualties.