Why can you not say 無貨階級 but 無產階級

Because estate 產 (a piece of land, whole property,class,stage in life) is a term more pertinent for social class while 貨 (merchandise 貨物 goods) can be dealt by all regardless "social class" just like a rich man could be a merchant but a merchant is not bound to be rich man.

these sentences will be corrected and put into my book. copyrighted!