We learn about this when Jonas meets the Giver for the first time and receives his first memory, that of sliding down a snowy hillside on a sled. The Giver explains that the community gave up snow and hills and "the rest of it" when they went to sameness.
The lack of hills and different types of weather means that life is a lot easier simply because it is more predictable and manageable. Transportation and agriculture are both more efficient as a result.
Just as we see the community adopting sameness in their clothing, their routines, and their rules, we now also see that they have applied sameness to the environment around them. This all but guarantees their routines will never have to change.