One: The Emperor, before he became a skeleton in a life-support machine, was male. He was also, according to what my son calls "the fluff" (ie the Lore as opposed to the rules of the game) the ultimate end of human evolution - Nietzsches' Overman and the Kwisatz Haderach rolled into one. He created his 'sons' - the Primarchs - from hs own DNA, but they got kidnapped, scattered across the Galaxy and by the time he found them, they had adapted characteristics based on the worlds they had landed on. Each of them went on to create a chapter of Space Marines based on their own geneseed. He is not recorded to have created any daughters. Possibly he found he couldn't get rid of that pesky Y chromosome from his DNA. Point being, that it could have been resolved by creating a female equivalent, a spouse or sister, to be Empress and create her own female Primarchs. But this was 30 years ago, and the main demographic for WH40K players was young males. If that remains the case, why change anything? If the numbers of female players have or do substantially increase, they have the option of introducing the Emperors 'lost' daughters and creating a whole bunch of new models. BUT, this is a niche market - there aren't many WH40K players of any gender. So without a clear mandate - such as demands by a substantial minority of active female players - they are not going to risk a move that may well alienate a lot of established players and fail to attract new ones to replace them. Niche markets are by definition small ones and a company that serves only such a market is aware of the fragility of their position and as such, risk-averse.

Two: Couldn't you have found an important issue to base your thesis around? This isn't an issue, it's a Quora thread!