Nitrogen is a lighter gas than CO2. Because air is 79% N2 they have about the same density, meaning if you fill a pit with N2, other air will mix in easily and the whole thing just becomes breathable air. Fill a pit with CO2, which is heavier and the process takes much longer, so it can actually be maintained if you keep adding a bit more CO2 to replace what drifts away over time.

It's still possible to make a killing machine that would use N2 instead but it would involve airtight pens for the animals and gas pumping systems, which would be much more expensive than a pit filled with CO2 that you can dip regular cages into. So, sadly yes, it's just about the money.