I agree with you that boys schools tend to fare better when turning co-ed. I think this is for reasons such as:
1. Some were quite prestigious boys schools and parents want their daugthers to attend because of kuodos i.e. Rugby, Stowe, Marlborough.
2. I think there may be a perceived benefit for a girl to be in a proportionally higher male environment as some parents may feel it gives their daugther the edge in the "real" working world.
3. Most parents of boys would not want to send their sons to a girl-heavy school as the curriculum at present is already female-biased and this would make it worse for their boisterous son.
...and before I get shot down in flames these are general perceptions, not everyones ideas!