Comment by andrewshadura

Or, if you put it differently: beta, being a "B", has two loops of almost the same size, the top and the bottom, almost complete (about ¾ of a circle). Eszett, on the other hand, is more like a bendy road, with no circle-like parts. The top bit is a U-turn leading to a wiggle that is not circular at all, potentially with a sharp corner (the remainder of the "z"). A long tail on the left side is still possible, since the long S had it.