I've talked to guys in the Reg who have seen some women try P Company. Well over 50% of the men who try fail it, and this hasn't changed much over the years.
The main issue, apparently, is that those who have tried were thoroughly unprepared for the full on, high intensity, wake up in a cold sweat screaming nightmare that is P Company. It's a sold 3 week beasting (2 weeks beat up and 1 week testing) that thins people out very quickly, as a result of injuries as much as lack of fitness/ bottle. There's screaming and mud and mountains and milling and it's all very emotional. None of the women who started - and good on them for trying - ever made it past week 2 as far as I have heard. Some people attend various 2 week Pre-courses, but it's not mandatory and, in some cases, can do more harm than good.
The Commando Course, though formidable, is longer and includes a great deal more preparation and 'build up' to the commando tests. You have five weeks (preceded by a three week Pre-All Arms Commando Course) to prepare people for the ordeal. They also allow re-tests on a regular basis. In many ways it's more 'scientific' and mentally easier than P Company, and the higher pass rates indicate that. It's a good course, don't get me wrong, just not nearly as Neanderthal as P Company.
As a result, you get two different types of people out the end of each course, which specifically suit the requirements of the organizations in which they will serve just fine.