Deleting answers is inconvenient for a few reasons:
Deleted answers, their histories and their comments are not viewable to users of all reputation levels. Users below 10k should be able to see this history.
Deleted answers are much more difficult to search for than undeleted answers. (e.g. if I want to figure out if a particular challenge was sandboxed). You have to manually wade through the pages of the sandbox. And if you blank them they can easily be filtered out of search results when you are not looking for them.
Leaving answers undeleted makes the sandbox more accessible to all users with the downside that there is some extra fluff hanging around. For this reason I choose to simply blank my sandbox posts when I'm done with them.
Its also worth mentioning that a deleted answer does take up more space than an equivalent undeleted answer, for users that can see them. To make an answer take up as little space as possible the optimal strategy is to blank the entire thing but not delete it. This is no longer much of a concern now that deleted answers are sorted to the bottom so they don't take up real estate, but it does justify why a lot of answers predating this change may not have been deleted.