I think it's because bullpens are no longer the domain of just "failed starters" but there's actually more nasty guys being developed/becoming strong relievers.
So the idea seems to be take the starters out before they get in trouble because they are throwing on fumes and bring in a parade of fireballers and other nasty guys to shut the door.
Used to be the Setup man and Closer were the only good relievers. So, starters went longer to try to avoid MR. Now those guys are getting decent in their own right...perhaps as a result of the "hard cap" 100 pitch counts these days, somehow has to pitch those innings, might as well get good arms and not just pitchers too bad to be starters.