I'd like to know what's reason for only 10k+ rep users can see all the deleted questions/answers.
I'm quite sure that the admins of SE had something in their mind when set this level.
The site is community moderated and most tasks can be performed by users with enough points. One of these tasks is to (soft) delete posts, to review these deletions and to undo them if needed.
To this end it is useful not to say necessary for those users to be able to see the deleted content, as explained in comments.
It thus makes sense to give access to deleted content to those users, and only to those users, that can (un-)delete the content. This is basically what is the case. One could argue that then one could distinguish between questions and answers, 10k/20k, yet this might be just too much effort for not much in return. Moreover, 10k was for quite some time the topmost level.
Actually, this last point might be relevant: deletion, and viewing deleted content, as the most invasive action then was given last.
Why it is 10k that used to be the last level, this is sure a bit arbitrary, but maybe not that surprising a choice for a number either.