Betting has nothing to do with the fact they've lasted 5 days. Everybody knows that the Umpires are rather strict on light now. That's no secret. That, combined with slow over-rates, means that we get less play in a day. TV companies have more to do with it than betting companies, if anything!
It is up to umpires and ICC officials to decide on an early start. I do not find any logical explanation to why they opted against it. The light has become a contagious issue since the power of decision came to umpires.
Hmm seems like you know all then can you provide a logical explanation to it?