A rolling eligibility year sounds like an administrative nightmare to me.

Lots of things run on calendar years (taxes, Alaska PFD eligibility, health insurance deductables, etc.) and, while in one sense it's totally arbitrary, it also means that the rules are clear and easy to understand and to administer. This means that there are fewer misunderstandings and errors in running a program and makes it more likely that everyone gets the benfits to which they are entitled quickly and without adminstrative screw-ups.