Why most of the strings factories gave up gut production in the mid of the 20th century?
I think there were two reasons for this.
Not the advent of metal strings, or modern music, these were consequences.
1. The working conditions were not sustainable any more
2. The factories had to invest huge amount of money for disposal of the waste material, due to new laws against pollution.
So, mainly for health safety reasons. Gut production, which was not easy at all, was also not economically sustainable.