Flammable fabrics, and a lack of fire safety education. The full stories of many theatre/costume/gown fires speak of people running around while on fire. The first instinct is to try to take the burning costume off, often not a simple task in the complex clothing of the time. A teenage ballet dancer in the 1800s, and the people around her, would have reacted very differently than we would today. Upon seeing the fire she would have wanted to get out of the costume, which would require her to run off stage before undressing. That is possibly the worst thing one could do. Stop Drop and Roll is no joke. It works. Grabbing someone and rolling them on ground, preferably with a heavy blanket or such, puts a fire out much faster than trying to get out of a burning costume. We teach kids about this so much that it has become modern instinct.
Historically people in colder climates (where they needed fire the most) would use wool, which is naturally flame resistant. As a careless kid I escaped getting burned at least twice due to this fact. First incident was when I held my socks really close to an open flame. Second when I overturned a space heater onto a neighbor's rug. In both cases the wool fabric carbonized slowly instead of catching fire. (Of course, the neighbor was not happy; but a house fire would have been much worse.) It is unfortunate that wool has become rarely used in modern society, as we now have to deal with the trade-off between flammable fabrics or potentially harmful fire retardants.
And today we're trying to get rid of many nasty chemicals used as flame retardants. To the point it has become a checkbox item for the eco conscious sofa buyer.
