Lemonade tastes great fresh, but it very noticeably tastes worse when canned/bottled. I think that disparity keeps it from being able to challenge carbonated sodas on a large scale.
In France you can get citron pressee in any bar, they just take a fresh lemon, ice, water and sugar (mix yourself how you like). Just keep some lemons in the fridge... it takes seconds.
Some big sit down chain restaurants in the US do a good fresh lemonade. California Pizza Kitchen comes to mind, and I'll often order it. But there is still all that fast food and canned/bottled/vending machine consumption to deal with.
http://www.youtube.com/watch?v=WY_JKz9detM