Question

.Why do you think the use of triangular prism, pyramid, and cone-shaped packages is so rare?

Answer 1

I think because of the triangular shape of it, that you cannot maximize or use the space for packages

Answer 2

Yep, that's basically it. Triangular prisms wouldn't be terrible, but pyramid and cone-shaped packages would be extremely inefficient. Look up the packing problem, and also the concept of tessellation.