Perhaps they don't have any AM arrivals. Back when I was a sucker and flew United, I would commonly walk between terminals at OHare to get the food I wanted. I"m guessing that they don't have many flights in the AM, few people connect between flights directly in that terminal and they likely have more traffic in the afternoon. Were there money to be made by having shops open there, I would think they would be open. That's the wonderful thing about capitalism. If there was demand, there would be supply.