Same reason why at the beginning and end of holidays, around here (and probably elsewhere) they block the outside lane where the passing lanes are.
When the traffic is heavy (and passing is pretty pointless anyway), having the traffic split and rejoin actually slows it down more than if it all just moves along at whatever speed it can manage.
Another "trick" people use heading North out of Wellington is to turn off at Aotea, go up past the Police College and back on to SH1 at the Paramata roundabout, now having right-of-way over the "normal" SH1 traffic. On heavy weekends the cops also block that off, as it really screws with the flow (if enough cars do it, SH1 comes to a stand-still as nobody ever gets a gap to move forward, constantly giving way to their right).
The rat-runs do mess with the traffic planners hopes and dreams; which they've usually screwed up enough already (let alone limits from budgets and nimbys) .
(SH1 ought to be two lane all the damn way. And lose the daft idea that main roads need to go through towns.)
Measure once, cut twice. Practice makes perfect.
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