Though the mythological, historical and other imaginary serials like Nagin may have a niche audience, they are also more expensive to make compared to conventional serials which are more related to real life incidents
For these serials, a lot of money and effort is required for the costumes, jewelry and other accessories, special effects required for battles and other events.
For example Mahabharat which was first broadcast a few decades ago, is still being shown on Epic channel.
So while these serials are broadcast repeatedly on the television channels, the producers should be very sure about their audience, because if they do not get advertisers, they will lose more money