It would be very simple to just say that the invention of the sewing machine by Elias Howe in 1843 made construction faster and therefore cheaper, etc. But, in reality, it's not construction of clothing that was expensive. To have a hand-sewn, custom fit gown made in the 18th century took only about 10 hours of work. And it was the only way to get clothing, so it had to be affordable to all. The mantua makers ten hours would be similar to many other trades worth of ten hours. A single day of work to afford having a gown made isn't much. What did cost a great deal was the fabric.

In order to, say, have a length of linen produced you had someone who planted and grew the flax, then harvested it. From there someone took it through a complex process of rotting away the hard outer stem so as to get to the fibers inside. Those fibers could then be cleaned and sent off to the spinner. From there the raw threads would go on to either a dyer or a weaver (sometimes dying happens after the weaving). If not being dyed it might be bleached after weaving. Then, the tentering process of making sure it's be washed (therefore shrinks) and stretched back out in all four directions. Since this is being done to long lengths of cloth it requires machinery and a building of great length as well (at least three stories). From there, finishing depending on the fabric type. Perhaps glazing or pressing. Printed fabrics add in a whole different step with block, roller, or hand-painting.

Just think of not only how many different sites that bolt of fabric traveled through, but how many skilled hands. What the industrial revolution does is simplify and speed up each of those steps so that textile costs begin to fall. Inventions like the Jacquard loom in 1801 and the Dobby loom in 1843 begin to add up. Chemical dyes in the later 19th century refine that process. Advancements in farming and harvesting, like the cotton gin. Even travel and transport becomes faster, bringing imports all around the world.

What all of this means is that our method of receiving clothing changes drastically. In the early 19th century most clothing is still made to order (excepting unfitted garments like shirts and shifts). By the end of it, catalogue sold and mass-produced sized garments are readily available. While this certainly makes things cheaper, what it does to quality is usually the opposite.

I've handled some turn-of-the-century low class garments before. They were well sewn, but the fabric was of a lower quality. Still sturdy stuff, just full of slubs and other defects. Heavily patched in all of the areas worn through by a laborer, it was clear that these garments had to last a long time. Even the lowered cost of clothing doesn't entirely make up for a lack of money. Maybe someone who knows more about the class changes throughout the 19th century could chime in, but I can't help but feel like the lower classes purchasing power might not have improved as greatly as one would think. I know the products changes, but the result of this is cheaper labor, therefore it seems a catch-22 in some ways.