Rory mentions oxygenation rate which is an excellent point but there's additional reasons why not keep ISS atmosphere at a lower pressure - thermal convection and air cycling. Pressure at roughly one atmosphere means that the ventilation system on the station works better and no pockets of carbon dioxide or even carbon monoxide build up, which would be dangerous to astronauts. The air is easier recycled / replenished and mixed with oxygen (electrolysis of water) and carbon oxides removed from it (Sabatier reaction). The ventilation system also works more reliably at higher pressure and its parts lasts longer between failures. Astronauts / cosmonauts also exercise quite a bit on the station to combat adverse effects of prolonged stay in microgravity on human body, so air pressure also helps them shed excess body heat. Overheating is stressful to the body, lowers performance and can be deadly. And they use all kinds of equipment that requires air cooling too, and it would definitely complicate biology experiments or even render their results useless.
Nitrogen is also relatively cheap to deliver to the station since it's not really a consumable within the life support system and is only lost at a low rate to its inefficiencies, and is also used for all kinds of other things both on the station as well as visiting spacecraft (purging atmosphere, non-toxic fire suppressant, ullage gas in storage tanks to provide fluid / gas pressure,...). So it would be delivered to the station anyway. But in theory, if it made sense from the logistics standpoint, it could be replaced with some other inert and non-toxic gases like, say, Argon. Especially if they would for some reason decide to keep the atmosphere at a much increased pressure, where nitrogen narcosis might become a problem. But they won't do that, there's no good reason tothere's no good reason to and the station probably couldn't support it structurally without coming dangerously close its ability to maintain pressure and not lose it to space.
