As Sian Massey-Ellis and now Welch have shown it is indeed possible for women referees to get to the high levels of football. Hence I don't think there is a conspiracy in the FA to prevent woman referees of reaching the highest levels (at least UEFA has shown it is a great way to get positive visibility for refereeing). But taking into account the previously mentioned statistic of 10% of referees being woman (I do not know how accurate it is, but based on my own experience it is somewhat correct, maybe even too high) in case of all things being equal there should be 10% woman referees in every league.
So the question is why the situation is what is it?
*Warning* from now on my message will include admitting that there are physical and mental differences between men and women that are effected by biology. If this is unbearable to you, please stop reading.
*Disclaimer* the whole theory is based on my own experiences and my rather limited understanding of biology, so I might be completely wrong. However, in my eyes this offers some ideas on why we have so little female referees on the top
To answer to that question, we have to look what it takes to reach high level of football refereeing. My top five would be:
-Skill
-Fitness
-Referee mentality
-Time
-Passion/ambition
The skill of the referee would not only include things like understanding the game and players, knowing the lotg and --- but also things like authority and knowing how to deal with players. Although the skills can be learned off the pitch to some extent you will need to get to the pitch and actually deal with the players and the game to learn to the full extent. Here we run to the first issue: mens game differs radically from youth and female football in terms of what the players expect of you. So to learn to referee mens football you have to referee mens football. If you want to go far you have to start early (I'd say there is not much point in promoting 40+ refs to any top league, whether male or female) and you (to my understanding) start from the bottom. So in order to became something in the mens game you would have to go to the park on Sunday morning to referee and start that at a young age. Hardly very appealing and requires a lot of love for the game. I'd argue that this is a one "gatekeeper" for women in mens game.
Second thing I'd put under the "skill" is authority. Although affected by many other factors the most primal authority comes from physicality. So muscle, height and tone of voice bring you authority. Women in general are shorter, less muscular and have a higher tone of voice when comparing to their male counterparts. In my opinion this is a thing that does effect their (or anyone else with the qualities mentioned before regardless of their gender). As a thought in regards to this: whom do you think could calm you down/prevent you from doing something stupid Nestor Pitana or Clement Turpin?
Then into fitness. Rather simple thing. Women are generally slower and weaker than their male counterparts (check any olympic results for example, the male top performers perform better than females).
Not necessarily a sex difference but difference in body composition, which is affected by sex. So shortly, it is harder for women to meet the physical requirements for top level football.
Thirdly referee mentality, certain mental toughness, resilience under pressure etc. Women are higher in
neuroticism (neuroticism=tendency towards negative emotions). So we can make the argument that the kind of "referee personalities" are rarer in women. Which would result in A) less women referees overall B) higher quitting rate of female referees C) lower number of female referees in the top.
At fourth place is time. Not only the time you need to invest in training and to games but also the previously mentioned starting early. It has been studied that men and women value different things in the
workplace (not a study in itself but it combines a few different studies nicely). So women tend to value work-family balance and other "soft" factors more than men. In order to reach the top in pretty much anything you need to make sacrifices and for example, have to spend time away from your family. Hence I'd say that there are fewer women willing to make those sacrifices than men. This would result in less women in the top (If you wont be willing to make the sacrifices to succeed someone else surely is and will succeed).
Fifth is passion/ambition. I do not expect this to have major gender related differences, although men are higher in
aggression (and ambition is a one form of aggression).
In conclusion, there are factors to success in refereeing that exist more likely on men resulting in less female referees in the top. Even though individually they seem rather small, when combined it has a great effect.
Lastly, someone pointed out having role models for women wanting to became referees. While I understand that it raises awareness of the possibility of becoming referee I doubt it alone will make anyone a referee or at least make them not quit after a game or two. Out of interest, how many of us became referees because we saw someone refereeing in the UCL?