Cost mainly. A featherweight drive system can be built from cordless drive motors and gearboxes. A typical rammer can be built for around £200 to £250.

For a heavyweight you are looking at motors that cost £200 a pop depending on what you go with. They generally need a drive train manufactured as well which isn't an easy thing to get right first time and can take many attempts.

Moving them around is also a bigger pain due to the sheer weight and increase in size. A single person can happily lift a featherweight around and work on it at all angles. A heavyweight needs at least two people to move around.

Whatever route you go down you will make mistakes. A featherweight means that these mistakes will likely be a bit cheaper.