Location is one of the biggest factors influencing shop sales
one of the main differences between thriving shops and shops with sluggish sales is location
「売上不振」= sluggish sales / a slump in sales / poor sales
「繁盛店と売上不振店の売上の大きな違いはロケーションの違いによるところが大きい」=Location is one of the biggest factors influencing shop sales / one of the main differences between thriving shops and shops with sluggish sales is location
「売上不振」は英語で「Poor sales」や「Stagnant sales」という翻訳があります。
「繁盛店と売上不振店の売上の大きな違いはロケーションの違いによるところが大きい。」には英語で「The biggest influence in sales between prosperous stores and poorly selling stores is location.」という表現が良いと思います。
例文
「高い税金と売上不振が同社を破産させた。」 High taxes and poor sales bankrupted the company.