So where did this recommendation come from?
Back in the 80's (that's when Duran Duran and Michael Jackson were cool) The W.H.O. (not the rock band) reviewed a whole bunch of sciency studies in to the rise of heart disease and other cardiovascular conditions, and concluded that eating MORE vegetables lowers your risk of heart disease.
From that they made the recommendation for people to increase their intake to at least 5 portions a day.
Problem is those "evil" (they're not actually evil) fruit and veg producers pounced on this recommendation and collectively created a marketing campaign, promoting the 5-a-day.
So whilst the recommendation was backed by the WHO (not the rock band), the idea was popularised by the fruit and veg producers in a marketing campaign.