The Pirates are succeeding because of their pitching (2nd in ERA, 19th in Runs scored). They're not particularly great at allowing walks (4th most) or striking out batters (11th most). It all come down to the fact that they don't allow many hits. They're #1 in lowest total batting average allowed, bases empty, runners on and runners in scoring position.
They're 3rd in lowest slugging allowed with the bases empty because they allow a fair number of home runs (7th lowest), but it's with runners in scoring position that they become truly extraordinary. The next closest in lowest slugging allowed with RISP is Boston at .348, which is an astounding 41 points higher (if you go 41 points higher than Boston, you'll get to Miami at #13). The Pirates have allowed the 14th most walks, but the fewest hits and fewest home runs.
Will it catch up with them at some point? Maybe, but we're already halfway through the season. It seems to be working for them. The might not have a lot of clutch hitting (#26 in hitting with RISP), but they seem to have plenty of clutch pitching.