Single Beam Gantry Cranes have been around for a long time.  Recently they have gained popularity, mostly because  the length and weight of manufactured bridge girders has increased.  These replaced two beam/hook machines as they needed to grow and become more expensive, many times more expensive than two smaller single beam cranes.  The single beam design is also more compact, and maneuverable, therefore more suitable for many jobs that in the past two hook machines were doing.  Also, with LP power, wireless remote control, and electronic all wheel steering, they have become an alternate to overhead cranes inside many facilities.