Second, the further we got into the 20th century, the higher prices for all of the needed materials got, till by the 80s it was no longer cost effective to make most things yourself. And now, with ready made patterns costing over 20 dollars, and good quality materials about the same per yard, and up, making a dress shirt at home could cost as much as a very well made RTW shirt.
Third, the insidious intrusion of branding into the RTW category of clothing, made having name brand things with obvious logos far more attractive to the average consumer. Additionally, the precision of finish given to average ready made garments is not something most home sewers can get to these days. And the last thing someone would want, is to be in clothes that scream home made. Conversely, having someone compliment you on how fabulous you look, and then to be able to say that you made it yourself, is a pleasure that few understand now.
And fourth, the tremendous energy of the Women's movement, especially in the 60s through the 80s, made the idea of the homely arts very unattractive to a majority of women. They were flexing their power in the workforce, and in government, and had little thought, or time, for such wifely pursuits. The whole idea of sitting quietly hemming a skirt, or arranging trimmings on a hat, stank too much of the old guard mentality for the newly emancipated women of the time. And who can blame them? After thousands of years of enforced servitude, women finally had the chance to explore the world on their own terms. With money to spend that didn't come from their husbands or fathers, they marched into shops all over the place and spent it.