My best guess is because you are still considered a new member with not much content associated with your account, as for the reason why they do this is probably to avoid spam uploads. As you may notice, it looks like you are set in the "Members (New)" user group. This may have some limitations until your account 'matures' more. If a staff member sees this thread, I'm sure maybe they can explain more in detail on how this works. However, from my knowledge, the more active you are (total content, days registered), the fewer limitations you'll have.