It's probably because the field of glasswork is not inherently known to the public to be a particularly masculine nor feminine trade. When you tell people about it, I bet nobody has ever said "Isn't that something girls do?" The thing keeping a lot of women from STEM fields is not necessarily the environment within those fields, but let's face it, little girls are dressed in pink and given dolls from the moment they are born and are often ostracized if they want to play with traditionally male toys. Same with boys, they are dressed in blue and given sports toys or toy guns and if they play with dolls people treat them like outsiders. This lasts through their entire lives.
Just last week I was out to lunch with three other male coworkers. One of them said to another "What, did your sisters dress you up like a ballerina as a child?" -- They are in their 40s. This just goes to show you how far into life these stereotypes go.
The answer doesn't lie in making boys' toys more girly or girls' toys more boyish. We have to change how society thinks if we want to truly allow for equality in genders. As you can see from every single Slashdot thread on this topic, we have a long way to go, or perhaps we have the wrong goal.