Scouting and Guiding originated 100+ years ago from the same ideas but have since the outset always been separate organisations.
In the early 90's Scouting were suffering from a decline in numbers so opted to open up their units to girls if they so wished to join, Venture Scouts had been co-ed before this but I can't recall the exact date.
One of the reasons stated for needing to go co-ed was at that point in time the majority of Young Peoples football and rugby clubs were aimed at boys and not mixed which caused the decline in numbers. There were not as many options for clubs for girls. Their popularity has soared again across the country since about 2007 and their centenary as they seem to have got a very good PR machine working.
Girlguiding belongs to a wider world organisation which at its very heart is the offering of a Girl Only Space.
The benefits of a Girl only space is that girls have been proven to be more adventurous, more open with opinions, willing to try new things without the opposite gender around. I have witnessed this taking a group of girls to a climbing wall - all gave it a go, no one became silly and scared, and all made it to the top. A month or two later same group of girls at a mixed gender camp barely got halfway up the wall as all the above appeared!
As others have said in some areas girls would not be allowed to attend if it was mixed.
I am another leader who does offer craft but not every week, and we also do just as many adventurous activites as our local scouts, but it is very dependent on the leadership team - we all have our areas we enjoy. I personally love craft and make a concerted effort to make sure I don't default to it. I do know units where it is not so well implemented.
I am a bit of a Scouting and Guiding geek coming from a very involved (since 1929) background!