Because a tower being down, does not necessarily cause an outage on some cases and it depends on that tower, from what I see theres no towers near that (red) tower that would give you coverage even if it was working. That could be a possiblity ??
OK so even before this tower was down, im guessing you were already experiencing issues and now that its actually down, your getting a nitch coverage? Is that the only closest tower towards your location? You usually need about 3 towers and be in a certain location within those towers to guarentee you some coverage. If I were you Id call Optus and do a coverage check just incase.