I paint on canvas, and the main reason I can think of is that there's no prep work to be done. You just apply oil to glass and done. With modern oil painting, y

I paint on canvas, and the main reason I can think of is that there's no prep work to be done. You just apply oil to glass and done. With modern oil painting, you usually buy stretched, prepped, framed canvases that are ready for use straight away, but in this lady's time or before it, it was more common to use raw canvas and do your own surface prepping, cutting and framing, meaning gesso prep and stretching it across the frame for just the right level of tautness, which isn't something you do by hand, but with a special tool. All told, a more complex, time consuming and pricey process than just laying paint directly onto cut, hard glass.