Over the last few years, the Pittsburgh Pirates have been plagued by lacking quality left-handed pitching (outside of their former closer). With the exception of Steven Brault and buy-low free agents, the team has not had any real impact lefties in their rotation or bullpen. While having both lefties and righties is not integral to having success, it definitely helps make teams tougher to matchup against.
The issue is that former General Manager Neal Huntington never really took the time to invest in left-handed pitching prospects. He rarely used top draft picks on them and never really looked to acquire them in trades.