@jamesey Well you never see manga where there's a real male Dom and female sub in a consensual D/s or Master / slave relationship (also known as power exchange relationships). But you see a fair amount with a naturally masochistic and / or submissive male freely expressing and engaging in such things willingly with a female or - in the case of BL - another male. Thus I suppose that in the industry it is faux pas or verboten to have a female freely express a natural interest in a power exchange relationship where she is the submissive, only leaving behind the option for "she's a slave but it's okay because he's so nice she enjoys it" loophole. Thus, you have both the slave trope and its popularity explained.
Furthermore, to underline this point, while its not a well liked book necessarily in the real BDSM community, 50 Shades of Grey sold hundreds of millions. However, an equivalent in manga has yet to be seen despite the proven easy money it'd be. One must ask themselves why companies, being entities supposed to be interested purely in profit, would not produce such an equivalent.