Answer
The jello has no tensile strength because it has been essentially disassembled and restructured.
Work Step by Step
When gelatin is boiled it denatures the primary ingredient, collagen. This means to essentially take it apart and rob it of it's properties, such as its tensile strength. When the boiled jello liquid is cooled it's pretty close to what collagen is actually like when it's initially secreted by the fibroblasts. However, to give collagen its tensile strength, the fibers need to be aligned, bundled, and cross linked. This tight netting of sorts is what provides its strength. In the gelatins cooled form, these fibers are jumbled and messy, so there is no strength.
