The design of the Constellation program called for two main launch vehicles, The Ares I and the Ares V. The Ares I was said to be human-rated and was to be used to carry the Orion vehicle and its crew to Low Earth Orbit. On the other hand, the Ares V was said to be purely for lifting cargo. Wikipedia provides an explanation for the reason behind this decision:
Unlike the Saturn V and Space Shuttle, where the crew and cargo were launched together on the same rocket, Project Constellation was planned to use two separate launch vehicles, the Ares I and the Ares V, for crew and cargo respectively. This configuration would have allowed the two launch vehicles to be optimized for their respective missions.
After the cancellation of the Constellation program, the Space Launch System started development. Its goal is to replace both Ares launch vehicles. The Wikipedia article on the Space Launch System notes this change:
The NASA Authorization Act of 2010 envisions the transformation of the Constellation program's Ares I and Ares V vehicle designs into a single launch vehicle usable for both crew and cargo, similar to the Ares IV.
Why did NASA choose to create a single launch vehicle instead of making two separate spacecraft? Why did they choose to go back to the conventional method of launching both crew and cargo in one launch? It seems that there would be many benefits to creating two vehicles. One for crew one for cargo. Was it for cost reasons, or was there more to it?