According to your link, a poll in 1973 shows 43% of students having tried it with 51% in 1971. That 1979 figure is for people who are currently using it. I suspect the percentage that have tried it, rather than the percentage of regular users, is a closer fit to the percentage who would have supported legalization back then.
Furthermore, even if the percentage was under 50%, it's clear that once they grew older they didn't exert the massive influence over marijjuana policy that would have been expected. If 30% or 40% of 25-40 year olds actively support something, even if they are not a majority, that's going to be very prominent in politics, and heavily drive the discourse, and that just hasn't happened. (And even 30% or 40% might be enough to pass legalization considering that a lot of the remainder are probably just neutral on the issue.)
It's not unheard of for people who've recently tried various substances to nonetheless support stricter restrictions on them. The usual narrative goes something like "I can handle this, but there are lots of people that can't, and we have to keep it out of their hands", though the people in question vary -- drawing class, demographic, or cognitive lines is common.
There can be other ulterior motives, too. In the early 2000s, a few marijuana growers in Northern California were among the opponents of a ballot proposition that would have legalized it in the state -- because legalization was expected to harm their profit margins, doing more damage than than removing the chance of arrest would have made up for.
Another month has passed and here is a new rationality quotes thread. The usual rules are:
And one new rule: