Regionals (called commuters) back in the 80's and 90's paid very little and usually had poor QOL, but the pilots would put up with it because:

1) They were entry level.
2) The industry had a tradition of dues paying.
3) It was all turbprops, and for every turboprop in the fleet there were probably 7-8 jet airliners...there were only a handful of t-prop drivers to compete for the bigger jet jobs. Do a few years (like 2-3) at a commuter, then move on to something else.

Today, some things are the same, but other things have changed dramatically:

1) Pilots are still entry level.
2) The dues paying tradition still exists (especially in the minds of older pilots who paid theirs).
3) Unfortunately, the RJ's make up a MUCH larger percentage of the domestic fleet, and they can do almost ANY domestic route. This means that:
a) There are more qualified jet drivers in the regional pool
b) There are fewer big jet jobs because the RJ's have replaced so many.

Management naturally took advantage of the situation to operate domestic routes with entry-level dues-paying labor.

Unfortunately the major airline pilots let it happen by failing to lock in scope. The likely reason is that they didn't want to have any major pilots flying for less than 737 wages, which would have been required for 50 seaters.