* Jupiter is a gas giant: It doesn't have a solid surface to land on. You'd be sinking deeper and deeper into its atmosphere.
* Extreme atmospheric pressure: The pressure at Jupiter's cloud tops is about 10 times that of Earth's atmosphere at sea level. As you descend, the pressure increases dramatically. This would crush any spacecraft.
* Strong gravity: Jupiter has a much stronger gravitational pull than Earth, making it extremely difficult to escape its clutches. Even if you could land, getting back into space would require a tremendous amount of fuel and energy.
Takeoff from a spaceship in orbit around Jupiter is actually relatively straightforward, as long as you have enough fuel to overcome its gravity. It's the landing that poses the insurmountable challenge.
