A square pyramid has 5 sides.
a pentagonal pyramid
It has a pentagon base and it is a pyramid
pentagonal pyramid.
Assuming that you mean pentagon, not pentagon, the answer is simply that a truncated pyramid has a flat top which can support weight easily whereas a pentagon pyramid would require a very fine balancing act.
6
A pentagon-based pyramid
A Pentagon Pyramid has six edges!
4
A hexahedron.
PENTAGON
Yes, it is.