Why is triangle stronger than a rectangle?

Answer 1

Given unchanging lengths of the sides, a triangle cannot change its shape. But given unchanging lengths of the sides of a rectangle, it can change its shape by some force by changing its angle measurements. If a 2d load were put on a rectangle, enough force could squish the rectangle into a parallelogram, whereas a triangle cannot change shape without changing the lengths of its sides or bending its sides out of shape (most likely into a curve).
Given these properties, a rectangle can collapse its shape much more easily and is flimsy compared to a triangle.

Answer 2

A Triangle is stronger because it distributes weight to the ground where a Rectangle distributes its weight to its edges.