What are some key reasons an enterprise should not move to cloud?

Some of many include:

They do not have a comprehensive and detailed cloud strategy.

They don’t know exactly why they are moving to the cloud.

They think that the cloud will be cheaper.

Internet connectivity might be an issue, it is after all, a single point of failure for all cloud services. A misconfigured update wiped Facebook off the map only a few months ago, same with Fastly customers a few month's earlier.

If you have systems and apps that need to be air-gapped for security reasons would be one reason.  Another could be that if the choice is Public Cloud, vs. in-house data center and you have extreme performance needs, or you have to use a specific or proprietary form of encryption that is unsupported in The Cloud environment.

For the most part, we are now at a point, where The Cloud is almost ubiquitous and fulfills the vast majority of needs and requirements for most scenarios.

I think in general there will always be areas/systems that will benefit from moving to the cloud or staying on prem.
Specific application requirements like ultra low latency or security requirements are a reasons to keep a system or application on prem

This is a bit extreme, but if you consider a validator node in crypto or NFTs, it’s the same kind of node as a co-location or a data center could be. No matter how big or small it is, it still has to be provisioned with compute and storage. The other services that we take for granted in enterprise IT — security, resiliency, etc. — are not even available in the cloud. That’s why I look at the small nodes, like a validator that a human being would want to set up to play in the crypto game. Because if I put that in a data center, I'd have security, robustness, and resiliency. I'd have the Five9 support and everything else I need.

And I can still make it cost effective. If I look hard enough, I can get a small enough server with a good enough set of components in it that will allow me to do whatever I want. That becomes my one node that I can then add that storage to, or add a server that allows me to upgrade and keep adding RAM and GPU, etc., to give me exactly what I need in my own personal node. That could be any enterprise application, wherever and whenever I wanted or needed. But I don't have that facility in the cloud because I'm relegated to the instances that they will give me. And I’ll be in trouble if that instance goes down.