Write a paragraph on the reasons for a lack of manned space probes to Saturn:

We don't yet know how to build spaceships robust enough to keep their crews alive for that long a trip, and that don't break catastrophically. That's partly what the ISS is helping us to do.

But if you need a longer anwser with more explanation

Let's look at it this way. The sun is at the center of the solar system, and we are orbiting quite close to it, compared to Saturn. If you toss a rock in the air, it will come down. If you toss it harder, it will get higher before it comes down. This is the same thing with the solar system (I am simplifying here, there is also the fact gravity decreases the further you are, but we can skip this for now). Now, we are on earth, how strongly (fast) do you need to toss something up for it to reach Saturn before it falls back towards the sun (being on earth is like being atop a building on earth, ti simply puts up higher to start, but that is it, we still have to break free of the sun's attraction). So, in comes the sling shot effect: by sending the probe on a carefully selected path, the probe will come from one direction and swing by in the opposite direction relative to the planet, with a change of velocity relative to the sun! Essentially, Venus was slowed down a minuscule amount, and the probe gained a lot of velocity. So, in the end, you gained enough speed to make it all the way up to Saturn, but since the speed you have to make it there is the same that it would have been if you had the rocket needed to make it directly, what you did is take a lot of time to gain that speed. So you did not make the mission any shorter, you took a lot of time to build up the necessary speed.