To answer this, just think about it in terms of the size of population available in each case.
If you have a district system, the only candidates that the voters in a given district have to choose from are people residing in that district. This means that there is a relatively small group of people from which the candidates can come.
But now think of an at-large system. In this system, candidates from the whole city or the whole state are eligible to run. This gives a much bigger pool of candidates to choose from.
An at-large system also gives you a broader set of candidates in terms of ethnicity and ideology. Where I live, there are very few liberals. On the other side of the state, there are very few conservatives. In district races, we are stuck with conservatives and the people on the other side are stuck with liberals. In at-large races, there are more choices for the people in both parts of the state.