The simple answer is The Industrial Revolution.
While I think that is true, the word ‘revolution’ assumes a quick change, not something that lasted over 150 years. So what happened?
Here is what I’ve managed to piece together:
In the colonies:
Before 1770, if a young man was not preparing the ministry, for college, he would be apprenticed to a craftsman to learn a trade, sometimes at as young as 11 years old. After as many as 7 years he would have skills and tools, a trade. If you remember Benjamin Franklin’s story, you know he disliked his apprenticeship and eventually ran away from Boston to Philadelphia to seek his fortune.
Especially after the American Revolution the system didn’t work very well. Many young men moved to new places, tried several trades and never finished apprenticeships. Housewrights and joiners couldn’t pass on their knowledge so easily. A man might need to teach himself the skills he lacked.
The pattern books of the period bear this out – their first plates teach basic geometry, knowledge a carpenter would have taught his apprentices as they worked. The books were very popular. One book - over 100 pages of geometry - was Peter Nicholson’s The Carpenter’s New Guide, published in England in 1792, and then in Philadelphia, PA. Published into the 1850's, it went through 16 editions.
The pattern books I have read (list supplied upon request) do not clearly spell out how to use geometric proportions and ratios to determine size and placement in design. I haven't yet figured out why.
An aside: Nicholson himself was self-taught. His biography on Wikipedia is fascinating.