Why is McMurdo Station in Antartica more hostile than Longyearbyen (Norway), while Longyearbyen is closer to the pole?
The lack of oceanic and atmospheric currents going into and around Antarctica from equatorial latitudes means that much less heat is transported from the tropical latitudes to Antarctica compared to Svalbard. Therefore, at equal latitude (i.e. equal solar isolation), temperatures in the south are much colder.

In Antarctica, at 75°S latitude most of the land has permanent snow cover.

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.