I'm going to risk being tarred and feathered here, but when I look at people who have a passion for things like Transformers, I see the same thing I see with Star Trek fans. The show was aired during afternoons and early evening when kids could easily watch. Just as people tend to have musical tastes that are fixated on what they grew up with, I think Star Trek became as popular as it did in part because it was really good at attracting young people who just held onto that love, which created an audience that was exploitable to new shows and new younger audiences. It's iconic in part for the same reason some people consider Scooby Doo iconic: it's what you grew up with.