1. I suspect's impossible to trade mark or or register the design - so let's assume you raise your brand profile and get a reputation for good quality food at reasonable price - your competitor simply (almost) clones your line of packaging and piggy-backs off all your hard work - since they're a big conglomerate they'll under-cut you by a couple of pence too.
2. In certain countries such packaging tends to be associated with low-cost 'basics' brands - so if you're offering a premium product, you're likely to struggle.
Actually, here in good 'ol socialist Sweden all of our groceries, with a few exceptions, used to look like that up until the end of the 90s. I remember as a kid eating cereals from a big white box with the word "CORNFLAKES" on it in big blue bold letters.
Umm, no they didn't. What you're talking about was a particular brand, Blåvitt[1], owned by the Konsum chain of stores. It was a very concious (and successful) marketing strategy, and not some generic non-brand.
http://xkcd.com/993/