Aluminum cases act as giant heatsinks.
"These are my personal preference for a number of reasons, including light weight, durability, and, most important, the fact that an aluminum case acts as a huge heatsink drawing off the heat built up within the case. The difference in internal case temperature between aluminum and comparable steel cases is as much as 4 to 8Â° Fahrenheit or 2.2 to 4.5Â° Celsius."
The entire concept as an aluminum case as a significant source of system cooling originated not in a laboratory or an engineer's plans, but with marketers. Any superiority of an aluminum case in cooling lies with improvements in airflow and increases in the number of fans; not with the function of aluminum cases as some giant heatsink. If you think about it logically, then this is clear. The case is not in physical contact with the hot parts of your system, so for the case to serve as a heatsink then heat would need to be transferred from warm air to the aluminum case and then dissipated. This is NOT efficient, and for it to happen to any appreciable extent then there would need to be very little airflow in the case. In reality, virtually all of the cooling of PC cases occurs due to air movement. Coolermasters and Lian Lis ship with a better stock fan configuration than most other mid towers, so they appear to cool better. The fact that they are aluminum cases is incidental.