They are in 5 states so yeah, my point stands.
No, your point does not stand. You stated a "small" portion of the USA! Those 5 states are over 65 million people. From one In-n-Out in California to one in Texas is 1700 miles. 1700 miles and 65 million people. And your point was:
Im not sure how a restaurant that only covers a small portion of the country could be determined to be the most popular.
All that aside, the answer to your question is this is a family owned business serving 65 million people across an area over 1700 miles long. Further, every single location will always have a line that is 10 cars long. Most locations have 2 lanes and 2 drive throughs.
They are popular beyond your ability to think.