Given is the page of a book which I was studying. I was trying to study the derivation of the wave equation in a lossy medium. As I have underlined a sentence that says, "most of the case in which wave is propagating does not contain the charges". Why is it so?
This is not true in all cases, but at least approximately valid in most lossy media. Note that even if $\rho\ne0$, it is only the gradient of the charge density ($\vec\nabla\rho$) that appears in the wave equation, so the wave propagation is unaffected by a uniform non-zero charge density. Even if $\rho\ne0$, its gradient is usually (but not always) small.
A lossy medium has non-zero conductivity, meaning charge carriers can get redistributed. For example, if at some point in time there is a large amount of charge localized in a region, the resulting electric field can quickly cause this charge to redistribute, until a zero charge density is reached.
Again, there are lots of counter examples, such as a pn-junction.