Why are sponges sometimes not considered multicellular?
Sponges are generally considered as colonial organisms because there is little cell specialization and little separation of function/role. All cells do pretty much the same thing; it looks more like a pile of individual cells than an actual multicellular organism. In reality it is a little bit in between.

In any case, what one wants to call multicellular or unicellular is a matter of definition and preferences. You cannot find the line between unicellular and multicellular because there is no such line that would not be very arbitrary and filled with special cases.

You can study a little more the physiology of sponges and then decide for yourself if it looks sufficiently like a multicellular organism or more like a colony of cells (a colonial organism).