They have said some difference in their official homepage:
Kubernetes native
Legacy queue systems such as RabbitMQ and Kafka were not designed from
the beginning to work in Kubernetes environment nor in a microservices
architecture. KubeMQ was written in GO in a small footprint container to
fit for Kubernetes with simple and easy deployment in a preconfigured
cluster.
Cloud-native integrations
As a cloud-native message broker, KubeMQ is simple to integrate with
most common CNCF projects including Prometheus and Grafana for
monitoring, Fluentd, and Elastic for logging, and Jaeger and Open
Tracing for tracing integration.
No admin
KubeMQ configurations such as new channels and groups created by the
developers in the code, saving the hassle of admin settings which exist
in the legacy systems.
Off the shelf
KubeMQ is built as an off the shelf product, saving the need for
specific experts to maintain the product.
But, I think RabbitMQ is more general for enterprise application, at
least it is following AMQP, has wide support by different languages and
frameworks.
Regards.