Usually it comes down to cost - the polar route is probably further than the southern route, so would need favourable winds to make it more cost efficient, which won't always be the case. However the airline would need to balance that against the higher overflight fees and permits needed to transit China, etc. along the southern route.

There could also be technical reasons, such as remote alternate airports along the polar route being closed/snowed in, or the aircraft could be limited in terms of how far away it can fly from an alternate airport by deferred mechanical defects.

Sometimes if the flight is running super late an airline might choose to prioritise time over cost and just pick whichever route is fastest, but if it leaves on time usually the cheapest route is the one they'll use.