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?
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.
Specific answers require specific questions. When in doubt, post the output of "/ip address print detail", "/ip route print detail", "/interface print detail", "/ip firewall export", and an accurate network diagram.