Why Not A Mostly Underground Metro?

Since the 'increase in cost' argument seems to be not quite correct?

The cost of construction came down to Rs 153 crore (Rs 1.53 billion) per km in phase-II, as compared to Rs 162 crore (Rs 1.62 billion) in phase-I, despite an increase in the share of the underground component from 20 per cent in the first phase to 27 per cent in the second.

Also, why are the per km estimates higher for Phase III? Shouldn't they factor in the fact that the cost has actually come down?

Rs 250 crore per year, or 45% of annual income of the Delhi Metro, go towards loan repayment to JICA. Good for the Japanese. They're getting more than they will in Japan.