The wording of 5/day is a bit misleading. I think they must just mean 5/day in the sense of 35/week - I have had no trouble uploading more than 5 a day as long as I'm under the 35/week.
As for why, they are trying to force contributors to prune their submissions, with the stated goal of reducing similar images. There is a lot of discussion around this, on the DT boards and here, and it isn't as simple as it first sounds. DT doesn't have a strict policy, and you can see that there are some types of similars that they seem to encourage (i.e. facial expressions and poses) and other that they want you to combine into sets (i.e. illustrations of balls).