The simplest answer is that one of the elite pitchers in baseball was having the run of his life in the midst of a pitcher-friendly season in which overall scoring is down yet again. But perhaps we should take a step back and realize that some of the credit has to go to what may seem like an unlikely source: Dodgers catcher Yasmani Grandal.
We say "unlikely" because throughout 2014, we heard that Padres pitchers simply didn't like throwing to Grandal. Andrew Cashner didn't throw to him once all season; by the end of the year, Tyson Ross and Ian Kennedy weren't teamed up with him either. That's partially because catching colleague Rene Rivera was having an excellent year and also because Grandal was needed to fill in at first for the injured Yonder Alonso late in the season. But still, the message was clear: Grandal wasn't necessarily a pitcher's best friend.
Yet with the Dodgers, Grandal caught 17 of Greinke's first 19 starts, including every one during the streak (one of the two games that Grandal didn't start was because he had a concussion in late May). Despite the presence of A.J. Ellis, beloved by pitchers for his game-calling abilities, Greinke has gravitated toward Grandal, telling The Los Angeles Times in April that Grandal has "done everything … I couldn't draw up a better catcher at the moment."
It's easy to see why. Grandal, according to Baseball Prospectus' measure that includes considerations for the ball-strike count, is baseball's best pitch-framer in 2015.
Runs added by framing, catchers
1) Grandal 12.2
2) Buster Posey 10.5
3) Jason Castro 9.1
4) Tyler Flowers 8.6
5) Miguel Montero 7.7
What that means is that Grandal has done an elite job of turning balls into strikes, and keeping strikes from going the other way. (By comparison, Ellis ranks 72nd out of the 87 catchers tracked.) It's not a coincidence that Greinke has a career-low 4.2 percent walk rate despite a career-low 44.1 percent zone rate, which is to say he's managed the rare feat of throwing fewer strikes and still issuing fewer walks.
The combination of a pitcher at the top of his game with elite control and baseball's best framer seems like a lethal pair, and it is. Going by the PITCHf/x strike zone, a full 7.45 percent of Greinke pitches outside the zone have become called strikes, good for 10th in baseball. (The top two are Chicago's Jon Lester and Kyle Hendricks, pointing to some very good work from Montero and David Ross.) Flipping that to the catcher's side, Grandal's 6.83 percent of "balls" received that turned into strikes is the best of the 16 catchers with at least 500 innings caught in 2015.
It doesn't hurt, of course, that Grandal is contributing on the offensive side as well, putting up a 161 Weighted Runs Created Plus -- meaning "61 percent better than a league average hitter" -- that is easily the best of the 26 catchers with at least 200 plate appearances.
When Grandal was made the primary return in the Matt Kemp trade last winter, it was clear he had some work to do to win over both the Dodgers fan base and the pitchers on his new team. Nearly two-thirds of the way through the season and slightly more than that through Greinke's streak, it seems clear to say all parties involved are satisfied.