Because some people still seem to struggle with the concept.
Posting limits suck for normal posters and in an ideal world we could do without them but they do serve some useful purpose:
They limit the impact of any spammers inundating the site.
The prevent people with too much time and no self-filter from completely taking over and / or trying to use the forum as a live-chat system.
They are a useful throttle for anyone behaving badly (i.e. where an outright banning would be inappropriate).
Normal posting limits are very rigid in that when you hit them, the post that you possibly spent a long time working on is gone (unless you saved it first). They also don't cater for the fact that people tend to post in bursts instead of at a very steady pace.
So instead, our limits are flexible and a bit elastic and when they do kick in they don't stop you posting, they just start to delay posts appearing - initially for short periods and longer, the more the limits are exceeded. If you have submitted a post and it doesn't immediately appear then if you wait, it will. It get's queued. The queue also helps to smooth out the system load so if a lot of people are posting at once then the queue may backup a little even though you haven't hit a posting limit (it also prevents anything being lost if I'm deploying a new version of the app / instances are restarting etc...)
But ...
If you don't wait then things go wrong. If you submit the same topic or post over and over in an attempt to make it appear then what actually happens is that you end up triggering the limits as well as posting duplicates and as a result the system delays your posts for increasingly longer periods. Even if I subsequently delete the duplicates, those still count towards your limits.
If you have posted something then please just be patient, there is no need to re-submit it and doing so will likely just delay it's appearance even further.