[Ellie May]

Some of the ingredients that they use aren't too appealing.
You can look up the definition for "by-products" online.
Some people just prefer not to feed them.
Then there is the preservative ethoxyquin (which they may be getting away from using now) that apparently is banned in some other countries. The box it comes in is labeled "poison". Of course the small amount used in dog food may not (or may) be a problem.
Then they use things like corn which some would say isn't digestible enough, etc., etc., etc.

So it is whatever the owner is comfortable with.