Usually in 1A there is a direct conparison with flights. With early booking, if rates are comparable (hard to find these days) people decide their preference. Not to forget the hassle to travel to/from airpot and early check in.
For my case, previously I travelled few times between NDLS-BSB via air, but due to recent prices touching 8k and more per person, I prefer 6k for my entire family in shivganga 1A.