Antarctica being a continent completely surrounded by water allows for specific climatic conditions. First of all, continental climate near the poles is often harsher than oceanic, since the annual changes in temperature tend to be very high. Antartica in specific gets really cold, due to no sunshine during the winter and the continental climate.
Secondly there is no land which hinders the circulation of water and air around Antarctica ("Screaming Sixties"). This circulation effectively cuts off most of air-flow across 60°-latitude from the north, thus preventing warmer air from flowing down south.
As for the flights, in addition to the fact regarding the distances mentioned by Erik, also consider that the only landing strip in McMurdo that can be used by planes with wheels is over an ice shelf (a floating platform of ice). Therefore, installing proper airport instrumentation is impossible, and landing is performed mostly by visual navigation. This is something that can not be done safely in the permanent darkness of winter months or in bad Weather.
There are scheduled commercial flights in summer from Punta Arenas to Union Glacier (latitude 80°S and more hostile than McMurdo). But they are extremely weather dependent. Most flights gets delayed, and waiting a week to fly is not unusual (it was even more common when the landing strip was some kilometers south in Patriot Hills). Therefore, an Ilyushin 76 plane is waiting in Punta Arenas the whole summer just to fly once a week at most. All that added to many other logistical complications makes the round trip ticket to Union Glacier about USD$22,000. It wouldn't be much cheaper to McMurdo, so you can imagine that few people will be able to afford it. Therefore it is kind of a niche business that is not attractive for commercial airlines.
The only landing strip that is on solid ground in Antarctica and is large enough to receive commercial planes (medium size, nothing big) is on King George Island (latitude 62°S). DAP airlines also has scheduled commercial flights there, but they are also extremely expensive. Tourist flow in winter is minimal; Auroras generally aren't seen there, and therefore there is little draw for tourists to visit during the winter.