ExploderWizard
Hero
I think it is far more difficult to convey a real sandbox in a published adventure, and this skews the way people think about it, until they have some varied experience under their belt.
I am not quite convinced of this. I have some products on my shelf that are great published sandbox modules.
B1, B2, B4, B5 are all setting based adventures with no actual storyline. They are all full of people, places, and things to interact with. The DM can create dozens of adventure hooks for these areas.
A more modern product of this type would be Frandor's Keep for Hackmaster basic. That book is chock full of adventure hooks, locations, interesting NPC's and other tidbits.
Writing products for sandbox style isn't a big problem. Learning how to use them requires more effort, trial, and error, especially by someone who has only had experience running more structured adventures.