Question

It is easy to compress sponge because

Answer 1

1. its particles are loosely packed.
2. its particles have weak inter-particle force of attraction

Answer 2

A sponge is a porous solid.

Although the sponge is a solid structure with definite shape and volume, the intermolecular forces are weak. 

The sponge has minute holes in which air is trapped an. When the sponge is pressed, the air that is accumulated and trapped in the holes is expelled out and this helps in compressing it.