In many ways, it's scarcity that makes things worth pursuing. McDonald's breakfast is never about the food, but it's about being sure to wake up early enough to get it, while still having enough time to make it to school/work. It's really more of a game than anything else. If it's just sitting there on the shelf at 7:39pm...will people really be lining up to continue their breakfast binge? This is, of course, ignoring the ridiculous notion that eggs, bacon, and sausage are appropriate "breakfast foods".
Breakfast, despite it's inferiority complex which has forced it to rebrand as the "most important meal of the day". McDonalds had managed to tap the market years ago, but might just be overreaching by selling it all day. There's a reason we don't have fireworks every weekend...eventually they just get annoying.