In the movie "The Martian", why couldn't the crew land on Mars to pick Watney up?
While there are several vessels available to both Mark and the Ares III crew, there are a number of reasons why the 5 crew of the Ares III mission couldn't just land and get Mark Watney:
The remaining crew are aboard the Hermes. The Hermes doesn't have the Thrust to Weight Ratio (TWR) to either land or launch from Mars. Hermes is designed to travel vast distances with a low thrust, high ISP ion engine. It's acceleration (according to the book- it's not actually stated in the movie) is 3mm/s/s.
Then there's the MDV (Mars Descent Vehicle), it's what the Ares III crew used to land on Mars. It remains on Mars, but with no fuel.
Then there's the Ares III MAV. The crew used that to launch from Mars. Technically they left it in orbit, but it doesn't have any fuel left (also, it's in orbit).
Finally, there's the Ares IV MAV. It has some fuel, but it's thousands of KM away from the Ares III site.
The next problem is speed. The Hermes (along with the Ares III crew) is going really fast. And as stated earlier, the Hermes doesn't change speed fast. It normally takes months for it to slow down enough to enter Mars orbit and because of the Earth slingshot, it's moving faster than ever.