Why does the PA28 only have one door?
Having one door is just a design preference which makes the aircraft simpler, and lighter to build. 

There are many other light aircraft with only one door. Besides Piper, most Mooney, Beechcraft, and Bellanca aircraft only have one door. Even the high wing Cessna 195, 206, P210, 337, and low wing Cessna 310 and 340 have only one door.