Why is a service in state not monitored?

Nothing is wrong. This is normal and usually caused by your configuration.

A service can be attached to an interface by automatic service detection or manual in requisition definitions.
If the service is polled is configured in poller-configuration.xml. You node has to match the filter rule in a polling package and the service should be enabled.

Note that polling is separate from data collection. The poller monitors a service for simple up or down status, creates an outage when a service goes down, and the outage can trigger notifications to be sent.

Data collection is separate and collects performance data from the same service bind on the interface.

It is possible to not monitor a service with the poller, like SNMP, and still collect data via that service, which is indeed the default configuration for SNMP.