Triangles are inherently strong and stiff .- which is why most bicycles are made with not just one, but two. The main frame, and the rear triangle - those two back to back look kind of like a diamond, which is why it is called a diamond style frame. Aka "top tube", it was the first modern bike frame. Because the diamond configuration is the strongest from an engineering perspective (using traditional frame materials), the bike can be as light as possible. Diamond style frames allow designers to easily integrate batteries in the frame for a stealthier look, more like a normal bike.
For those of us who are either not quite as agile as we used to be, or just don't like having to swing a let up and over the tope tube - it is limits how fun riding a bike can be. On eBikes, the weight penalty can also easily be paid for by the motor and battery.