Because it doesn't have a fan as per its description, unless you buy an optional fan. Did you buy and pay for the optional fan?
http://routerboard.com/RB1200
RB1200
The new and affordable rackmount router.
It has ten individual gigabit Ethernet ports, five of them can be connected together in one 5-port switch group. RB1200 has a SODIMM slot with bundled 512MB of RAM, a beeper and a serial port.
It has no moving parts and it’s operation is completely silent, optional fan header is available. The RB1200 comes in a 1U aluminium rackmount case.