It was a direct result of the two Eagle accidents in one year, 1995 - RDU J32 and Roselawn ATR.
It was publicized that commuters had a "different level of safety" - that being 135 and 121, so Congress stepped in (and of course common sense went out the window) and asked for "one level of safety."
The most ironic thing about this was that Eagle was already 121. They had discarded all 135 operations earlier that year.
Part 119 was used to "transition to one level of safety" so pilots of smaller aircraft could still fly more hours, i.e. 19-30 seaters could still do 1200 hrs/yr, 120/month and 34/week, as opposed to "true" 121 stds of 1000, 100 and 30.
Like most rulemaking processes it was horribly political and did not really do anything to increase safety, but did increase paperwork by requiring old 135s, now 121/119s to have dispatchers.
Unless someone can educate me otherwise, the work rules are essentially still the same since the 119 change.
One positive note, thanks to NASA, a few years ago the "legal to start, legal to finish" rule was blasted out of existence over huge opposition from the RAA and other airline agencies. They claimed it would require them to write new software for their crew tracking programs (debatable!)
But now the duty limits for a day are hard limits. You cannot be on duty more than 16 hrs at a 119 carrier. Believe it or not this was not always so - I remember one Eagle crew sitting in the lounge waiting to do their last turn to Punta Cana, which had been delayed 7 hrs already. They had been on duty for 19 hrs by 10 pm and they really wanted to go home, but Crew Scheduling said "Legal to Start, Legal to Finish!" and would not let them go. They finally called in "Fatigued" and went home. You don't want to use the "F" word in airlines, as these guys found out. The next day they were subjected to interviews, doctors examinations, and all kinds of harassment just because they used "Fatigue" to "get out of work" supposedly.