The better question to ask is why does the controller not report it to the WCS via a trap.
Do you see it in the trap log on the controller? If not focus on an issue with the controller. If you do see it in the trap log then check the trap controls on the controller to see if it is set to send a trap to the trap receiver for that issue.
On the controller go to Management > SNMP > Trap Controls > Cisco AP Traps.
The last thing to check if it does appear and the controller is configured to send the trap to the trap receiver ( and WCS is added as a receiver ) is the severity on the WCS for the alarm being to low to report:
Administration > Settings > Severity Configuration