Hmmm, interesting. But I think the comparison should be between margins for a call-calendar and a straddle-calendar. The margin requirements for both should be far less than those for the single-month parts of the strategies. Ursa..
The short answer is that in a reverse calendar, you are expecting that after the news, the time premium in the short (far) month is going to drop more than that of the long (near month). That can be achieved in two ways. The vols can contract or the underlying's price can move away from the strike, driving the options toward parity. The more you get of each, the sooner you get into the plus column and the more the profit will be (a larger portion of the net credit). I'll leave it to the pros to provide the sophisticated yabba dabba alpha-beta-gamma technical explanations