What we know, based on that statement, is that if retailers do not excel in convenience and they do not excel in variety of merchandise, then they tend to not be successful.
Please keep in mind that we know nothing about retailers who do excel in one, or even both, of those conditions. In other words, do not assume that retailers who excel in convenience and variety of merchandise tend to be successful. Also, do not assume that if retailers excel in convenience, but not in variety of merchandise, then they tend to be successful. The only thing we know is what we've been told about retailers who excel in neither; they tend to not be successful.
A key to doing well on these logical reasoning questions is to not assume anything other than the information you are given in the stimulus.
I know you didn't ask for all of that, but I have noticed this as a common mistake, so I figured I would bring it up.