Both crossplane[1] and CNAB are attempting to play in similar spaces, understanding that deploying an application for the cloud (public or private) is more than just dumping your software into an image and giving it to a provider (be it K8s or some other IaaS/PaaS stack). There's more associated with the application, specifically what it means to orchestrate IaaS/PaaS/SaaS to realize your application.
But I see CNAB falling into the same trap as Helm, and many of the package managers before them (including newer variants with things like charm/juju), an archive with some notion of lifecycle events is not enough. Even though it's cute that lifecycle events are encapsulated in containers making it easier to manage their runtime dependencies.
What I think makes Crossplane's model more attractive is the notion of building on top of Kubernetes design and leveraging things like the operator pattern[2]. Now application stacks can do more interesting actions during their lifecycle, which can work to preserve availability during an event (like upgrading your application stack). Crossplane is about expanding the management of resources beyond just containers running in your cluster, but to any resource you can model in the K8s control plane and writing software that can react to events related to those resources.
[1] https://blog.upbound.io/introducing-crossplane-open-source-m...
[2] https://coreos.com/operators/