Nitrided is cheaper for the manufacturer, does not affect accuracy, and is likely to be at least as durable as hard chrome, possibly better. Hard chrome can have slightly greater surface hardness (RC70 vs. nitriding being anywhere from RC60-70) but nitriding never flakes off, which chrome can do. Also, nitriding is typically on all metal, not just the bore, so it gives the same hardness and corrosion resistance to the outside of the barrel.
All in all nitriding is a better process and I expect that it will displace chrome lining over time.