Because Luke finding the smoking corpses of his stepparents who were burned by stormtroopers, Vader torturing Leia & Solo and choking everyone who failed him, Tarkin blowing up a planet and killing millions of unarmed civilians in the process is totally for kids? Let's be realistic.
I agree that stuff in Ep.VI, the prequels and the crap added by the special editions is quite childish (although the CGI series take it to a new level) but that was the result of Lucas changing his intention from telling a story to special effects, humour and selling toys in order to make more cash.
Kurtz also explains how Return Of The Jedi would have ended originally, in the outline that Lucas and he had come up with before Lucas decided to change everything to make it more of an upbeat toy-selling vehicle. Luke and Leia would have rescued Han Solo from Jabba the Hutt, but then Han would have died halfway through the film, during a raid on an Imperial base. (This is something that Harrison Ford has mentioned before as well.) The film would have ended with the rebel forces in tatters, Leia struggling with her new duties as queen, and Luke walking off into the sunset alone, like Clint Eastwood at the end of a spaghetti Western. It would have been a more nuanced, muted ending to the saga, instead of the Ewoks dancing in the forest like a "teddy-bear luau.
I know that when i was very young, i liked the phantom menace more than empire strikes back because of JarJar's slapstick and the actionscenes (pod-race, anakin destroying the control ship) wheras in Ep.V there were things i didn't understand and thought it was boring.
It is quite clear that George Lucas focused on a young audience while the films produced with Gary Kurtz had a more mature tone.
In comparison to Star Trek, i think Star Wars was originally a much darker and oppressive Sci-fi universe.
Instead of a crew exploring planets with a mission of building up relationships and mutual understanding, we had people trying to evade and resist a all-powerful dictatorship which stopped at nothing to sustain its dominance