For your own deleted uploads it should also say:
If you don't understand why your upload was deleted, you can ask for advice in the Upload Feedback Thread on the forum.
Which means you should be asking this in topic #70 "Upload Feedback Thread (post here if your uploads are deleted)"
These kind of threads get closed quickly because there is already a pinned thread to concentrate all complaints/reviews
When you submit a post as a member, it is shown in the moderation/posts queue to be reviewed, up to 3 full days. If that period expires, the post gets deleted (can still be found browsing status:deleted). It will also show you how many approvers reviewed it, and any messages they may have left as rejection reasons.
As for the rejection reasons, every approver is by default given 3 options to reject - "disinterest", "poor quality" and "break rules". The default rejection reason is "disinterest", which usually means none of the approvers who checked it liked it enough to approve it (or thought it was not good enough)
You can still use the "Appeal" button on posts that are deleted to give them a 2nd chance, but this is recommended only after a while (to increase the chance that it is not the same approvers seeing the same post again)