Girls do not seem to have a usable head voice - the voice they use is a chest voice even at a young age and while it may deepen with age or even break it is not the extreme break of a boys voice when the vocal chords drop.
Cathedral choirmasters are an odd breed (as are Precentors). Some argue that the music was written for a particular (boy's treble) voice and so it should be sung by them. Others are more enlightened but there is an attitude problem with members of the church hierarchy. ther are also practical problems - most Cathedrals have a choir school of some description attached to them with subsidised places for choristers, as the voices don't mix it entails running to choits which is very expensive. Also to achieve the high standard of the choirs requires a lot of singing and performance, if this is split across two choirs you can end with neither doing enough. The plus side is that with two choirs the boys do at least get the odd weekend off and are better able to cope with academic pressure.
To put that into context - my son is a chorister at Lincoln. His normal week involves:Monday-Friday 8.00am practice til 8.50am then school;4.00pm practice til 5.00pm except wednesday; evensong Tues&Fri: Saturday 3.00pm practice followed by evensong; Sunday 9.00am practice followed be Eucharist, Matins,lunch, practice and Evensong! The girls choir at Lincoln does one Sunday in three and Tues/Thurs Evensong. Christmas and Easter are worse!
The positive side is a superb musical education, loads of performance experience and bags of confidence and some superb music!
All the best