that's since non AC travel in IR is too subsidized. IR staff costs have increased after Newer pay commissions like 5th pay, 6th pay, etc while ticket prices have increased but not in proportion to increase in costs.
IR doesn't want to do a significant increase in non AC tickets, so it is encouraging passengers to switch to AC economy from SL class.
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If IR wants poor people to shift, it must CAP AC economy fares to ONLY 2 X times the sleeper fare. I don't know if it will happen.