Because he wasn't? Bismark united Germany by waging wars against Austria, Denmark, and France. Once he gained the territory he wanted, he was careful not to punish these nations. He could have pulled a Napolean and taken over all of Europe.
He made it clear he was just trying to unite his people, not take over the world, and under Wilhelm I, this system was stable.
All of the major western nations were undergoing industrial militarization at this point, particularly with naval technology.
When Wilhelm II came into power, he removed Bismark whose sole goal at this point was managing peace in Europe.
At this point all of the tension was in the Balkans, Wilhelm was eyeing new conquests, the Serbians murdered the Austrian heir to the throne, and every major power thought they were God's chosen people. They also thought any new conflict would be short and sweet just as the recent Prussian wars had been. No one understood how new technology and tactics would change the dynamic of war.
Bismarck was chancellor until 1890 and died about 10 years later. WWI didn't start until about 25 years after he was no longer running the show. People had moved on from his notions of European peace and wanted to play with their new toys.
Additionally, the chancellor was elected by the nobility. It was not as if he could have handed the role down to a successor. He would have had to find someone that appeased the new king, a younger nobility, and the public at large who were quibbling over democracy vs a semi constitutional monarchy.
Bismarck's talent was uniting people and keeping them happy, even when not everyone got what they want. He was respected by his enemies because he illustrated that he was just trying to unify the remains of the Holy Roman Empire, a gelatinous blob of bickering principalities that had been central to European conflicts for hundreds of years. Unification finally stopped the bleeding, and Bismarck made it clear that he was after global stability, not conquest.
If you want to blame someone for WWI, there's not really one person you can point to. However Wilhelm II would fit that mold much better than Bismarck. If he had been alive and in power, he would not have wanted to start a massive war of attrition.