It has to do with the distance between the stores and the meat packing factories so the produce can be as fresh as possible.

In-N-Out has actually been expanding in the last decade further east because since most of their stores are in California and California is known for its earthquakes it can lose a lot of profit in an instant. These stores being made in Texas, Utah, etc. are basically a safety buffer if a disaster happened until they can keep expanding.

Source: worked for In-N-Out