I would think ease of care would be the best answer. In my opinion (I definitely DON'T have a green thumb) it's alot easier with work/school to spend 1.5 hours a week cutting the lawn then many hours weeding a flower bed. I do have some small flower beds to add colour to my yard and use lots of mulch to cut down on the weeding.
It can also cut back on the amount of water being used. In the summer I don't generally water my lawn (unless we're going through a really bad dry spell, then I might water according to my municipality restrictions), however, flowers tend to need more regular watering to keep them in good shape.
Mainly because of the costs and the upkeep. A plain lawn is cheap and simple to keep alive and maintained. A huge flower garden requires more money, more work, and lots of time spent maintaining it which isnt very practical for most people.
Flowers around is so refreshing and i do love flowers a lot. We have a garden with lots of flower varieties
U don't have to mow the garden and gardens r more beautiful than grass :-)
It's a LOT more work?