A triangle is a truss, which is an inherently stable structure because all sides support each other in tension and compression. Any external force is distributed across all three members. As long as the lengths of the three members are fixed, the structure won't change shape, so it's inherently very stiff. In comparison, in a 4-member structure (like the Clean bike where you have the seat tube, chain stay, seat stay, dropout) both the angles and the lengths of all members must be fixed for it to retain its shape. The lengths of the members are inherently fixed, but the angles are not unless there is some kind of bracing. In this case the welds are what prevent the four-sided structure from deforming, so I'd assume the welds experience greater stresses in this configuration.