Dozens of airports have past or present aircraft movement limitations, in lots of different countries, so it is hardly unique to SYD! As for the OP's somewhat rhetorical question, the answer is almost always the same: it is part of a noise abatement or pollution abatement plan! Capping plane movements is not the only way to accomplish the same thing; LAX is a great example, where the plan has not capped aircraft movements but instead has capped gates and even car/bus movements (forcing deliberate traffic bottlenecks and consolidation of shuttle bus service). Which is less convenient for pax is a good question.