I'd say that it's because teenagers tend not to stick to anything (even being a teenager). I know I didn't, I had lots of great ideas and opinions that I changed either because they resulted in too much bother or because I grew up.
With me, and there was no "social media" in those days, it was platform shoes, flares and long hair. I can imagine that if vegetarianism/veganism had been fashionaable then, I'd have joined in. Teenagers always thing they know everything, are full of moral certainty and "passion" but then they get some sense.