Because nitrogen is already the dominant gas in breathable air. Because it is plentiful and readily available. Because lungs don't detect it as anything different than regular air. It has no side effects and no toxicity. It isn't the nitrogen that kills, it's the lack of oxygen. What humans detect when suffocating is excess carbon dioxide, so as long as that'a removed from the nitrogen enviroment, people will just blissfully slip into unconsciousness and then experience brain death from oxygen deprivation - while in a euphoric state the entire time it happens.
The article title is sensationalist clickbait at best, and outright disinformation at worst.
Idk I looked it up (the claim that vets don't use it) and what I read was that it can cause feelings of suffocation. The vet claim here was a little challenging for since I doubt that pets would like a mask on their face regardless of what is coming out of it.
But is oxygen cheap? Oxygen is easy enough to filter out of the air, but storage and shipping are not simple. By contrast as you filter out oxygen, you're leaving basically pure nitrogen (sure with argon and the other various gasses in negligible quantities.). I truly don't know. Would be happy to be educated one way or another.