"Only fear the Lord and serve him faithfully with all your heart. For consider what great things he has done for you." 1 Samuel 12:24

It's really tough to only do one thing. But there is a purity to simplicity. A sole focus pays dividends. Think In N Out Burger. I'm sure they've had plenty of opportunity to diversify. They focus on what they do well. When God says only fear God and serve Him, He bases it on what He does well. He gives a call to remember everything He has done for us.