Comment by jacquesm

Because the batteries have to ride somewhere, in a bag in the back it messes up the weight distribution of the bike and every time you make a turn all that weight bangs into the frame at the back. Another problem is that you get a silly little bit of range per battery, 30 km on a good day, so you end up maxing out those batteries. If you want to spread the load across four batteries (same weight) then you will have to swap batteries every 16 km or so. So this is far more practical and much better for the cells in the longer term as well as a safer bike to ride.