Solids are those substances in which particles are very close to each other. Solids cannot be compressed because particles of solids have very less distance between them and this distance cannot be decreased. Liquids are also less compressible. The distance between the molecules of a liquid is greater as compared to solids or very less as compared to gases. The gas particles have a very large distance between them. The volume of particles of the gas is negligible as compared to the volume of gas. Gases can easily compressed because of the distance between particles of gases. In other words, gases can be compressed because they have less density and solids and liquids cannot be compressed because they have large densities.
Because the molecules aren't as dense. In liquids the molecules have some room to move around. In solids there is almost no room. In gases the molecules are moving around very quickly and spaced out. Does that answer your question?