I suspect they're also taking advantage of the fact that dry ice is solid, so they're effectively sand-blasting the track (rather than merely pressure-washing it), but they're using sand that helpfully sublimates into the atmosphere without leaving residue (or poisoning anyone).
I imagine blasting sand or chemicals has the downside of leaving residue that can build up, meaning regular cleanup is required, and care required to use enough but not too much, with this you can spray as much as you can afford to buy/carry on the train and you don't have to worry about over-doing it.
Dry ice blasting is already a thing, so I guess they just realised it was a particularly promising technology for this application.
The use is novel, the tech is decades old.
LN2 might also then be too cold and affect the temper of the steel, or it might evaporate too quickly, requiring a lot more mass than co2. Or maybe it could be a perfectly fine alternative but the co2 pellet systems have market advantage.
Sand density: 1520-1680 kg/m^3
LN2: 806 kg/m^3
CO2 is just disappearing sand as far as these systems are concerned. They probably use the same equipment that they use for sand.
Plus LN2 is way colder and harder to handle and also it's a liquid.
Nitrogen will also kill you silently, while you will definitely know when CO2 levels are getting high (because it really really hurts).