I dont have an answer, but could be for same reasons why hospitals keep it on the cooler side: Less fungal / bacteria growth. Less mites, insects, bugs, etc.
Anyway, here is my guess for the two questions:
1) The recommended temperature in the lab is based on other factors than the mice's prefered temperature. E.g. comfort of the scientists and technicians, spread of bacteria etc.
2) The temperature prefered by mice was determined by studying them in the wild or in the lab. E.g., if given the choice between two places of different temperature, which do they pick?