All of NASA's original planning was all about sending more food and supplies to Mars, to keep Watney alive for years before Ares 4 was scheduled to visit Mars. In particular, they needed to launch some supplies to Mars very soon or Watney would starve.
They rushed their own launch but botched it. Fortunately, the Chinese had 1 rocket capable of making it to Mars.
And then... Rich Purnell figured out a plan to slingshot Ares 3 off Earth's gravity, and send it back to Mars, and pick up Watney directly. Big problem: Ares 3 was out of food and other critical consumables, so sending it back without resupply would kill the entire crew and Watney too. So to do the Purnell plan, Ares needed resupply from the earth.
So that is why Ares 3 met the probe.
To answer your question.
Second problem: the slingshot maneuver is NOT an Earth orbit, and is much faster - a higher delta-v. That means common, normal spacecraft like Soyuz or Raptor that go to LEO all the time, would not be fast enough to catch up to Ares 3 to hand off the supplies.
So they needed a Big Forking Rocket. And the only rocket large enough to carry that much resupply payload, to that delta-v, that would be ready in time, was that same Chinese rocket.
Thus they could not do both missions at once. They only had 1 rocket. They had to decide to either resupply Watney, or resupply Ares and send them back for Watney.