Because "new players" means open-minded students of the game, and unlike many veterans of gaming, are more apt at learning complex and complicated possibilities through experimentation and intuition.
My pre-teen daughter who has never played a PC game except some web browser flash based games at public library's has started playing league recently, and she's already executing full-combo's on Ahri and Lux with no direction except "Cool! Can you do it faster?"
In a few months, she'll probably be kicking my Gold ass.
Someone telling her "No, you shouldn't play that, its too hard for you." is exactly the kind of shitty mentorship that convinces people to settle for mediocrity and I really hope you never use this style of tutelage on anyone.
New players are kids and adults from all kinds of levels of experience and skill, and exposing them to a wide range of possible options doesn't condescend and undermine their learning curve. Every champion in league has their own nuances to learn, and there's no better avenue to mastery than trial by fire. Getting your hands on something cool and attractive is a lot more useful towards recruiting and retaining new players than something "simple and easy".
Also, you're in no position to be a reliable judge of what is or isn't beyond others' capabilities. Zed may be "simple and easy" to some where Sona is not. That's just how it is sometimes.