There are several reasons why this is an important issue.
First, people would be able to get into and out of gas stations safer and more efficiently if everyone was entering from the same direction.
Second, people who have multiple vehicles, or who rent vehicles often would always know which side the tank was on.
Third, and most important is safety. People are constantly being approached as they fill up. People-ESPECIALLY THE LADIES-have a much better chance of jumping into the car and locking the door if the filler neck was on the drivers side.
This would take an engineering change by automakers that sell cars in the U.S. but should be easily achievable by the 2020 model year. They have done much more complicated things in less time.