Because most people are completely unaware they exist and when they do hear about them at all, it's generally followed by a dozen people yelling about how they suck because they're not D&D. The perennial favorite of "rules light games can't do campaigns" is particularly maddening as it's most often repeated by people who've never actually tried. So, seemingly as always, we're back to Snarf's "it works in practice but doesn't work in theory" line. Some of the best campaigns I've ever played or run were with rules light systems.
Which is a vanishingly small fraction of the wider community.
Because they are effectively unknown by the vast majority of gamers. We also have people who torpedo lighter games because their preferences are for heavier games. As we see in this thread, multiple times. Mostly from people who either don't play lighter games or have decided beforehand that they suck so are oh so surprised when they do finally play a lighter game that they had a bad time.
Exactly. It's a miracle anyone's playing something as complex, time consuming, and crunchy as D&D when streaming and video games exist. If people were exposed more to lighter games, without the constant drum beat of "they suck" from some gamers, they'd likely do quite well.
The benefit of just being able to sit down and play without prior knowledge is wonderful. It's odd that people build up RPGs into this big important thing that must be taken seriously and must be played for decades to mean anything at all. Just because it's quicker and easier doesn't mean it's bad. Playing RPGs is only ever as hard as you make it on yourself.