Why doesn't Jupiter increase the chances of an asteroid to strike the Earth? I often hear the hypothesis that Jupiter decreases the chances of an asteroid strike on the Earth, but my question is, why does it only decrease the chances ? 

While Jupiter can deflect some asteroids that were originally going to hit the Earth, it can also deflect some asteroids towards the Earth. How can we be sure that Jupiter only decreases the chances of an asteroid impact on Earth ?
With regard to collisions with earth, or inner planet collisions in general, you're right that Jupiter has both a positive and negative effect on asteroid and comet trajectories. On balance though, the effect is mainly positive.
From earthsky.org:

Some astronomers believe that one reason Earth is habitable is that the gravity of Jupiter does help protect us from some comets. Long-period comets, in particular, enter the solar system from its outer reaches. Jupiter’s gravity is thought to sling most of these fast-moving ice balls out of the solar system before they can get close to Earth. So long-period comets are thought to strike Earth only on very long timescales of millions or tens of millions of years. Without Jupiter nearby, long-period comets would collide with our planet much more frequently.
...
But Jupiter creates both good and bad conditions for earthly life. Consider that its powerful gravity prevented space rocks orbiting near it from coalescing into a planet, and that’s why our solar system today has an asteroid belt, consisting of hundreds of thousands of small flying chunks of debris... today, Jupiter’s gravity continues to affect the asteroids – only now it nudges some asteroids toward the sun, where they have the possibility of colliding with Earth.