One reason is that Maven supports parallel dependency resolution, which is currently a feature request GRADLE-2582).
If you have many dependencies then it becomes worthwhile having an on-site artifact repository. That provides a caching proxy and minimizes disruptions if the remote repository disappears. I have gradle fetch only from local Artifactory, which has all of the remote repositories proxied. The builds went from 15 minutes to 30 seconds. Slow dependency downloading became a non-issue at that point.