Labor Unions were obviously a big factor and that link does a great job documenting that. But why did unions at that time choose to bargain for a decrease in hours worked instead of pay increases?
Because wages had risen sufficiently that workers valued an increase in leisure time more than they valued an increase in income. It required productivity rising to get to this point.
Unions bargained for defined-length days because longer days were killing people through both physical grind and the inevitable failures of (non-negligent) attention that happen when one is worked to the bone.
Instead of speculating about the reasons why workers wanted defined work hours and work weeks, why don't you use the many free resources at your disposal to either read primary sources or a summary someone wrote up?
That's how I know that the labor protections brought up by OP are not solely the creation of organized labor but also part of a long term trend of increased wealth and productivity due to technological advancement.
Because wages had risen sufficiently that workers valued an increase in leisure time more than they valued an increase in income. It required productivity rising to get to this point.