Is there a reason to use Terraform instead of flux bootstrap?

If you don't want Terraform managing Flux, the CLI bootstrap is probably the simplest fit. It's a one-time setup, and you don't need to add Terraform state for Flux just to get it running.

Terraform makes more sense when you're already using it to provision the cluster and want a single workflow for bringing up the surrounding infrastructure too. That convenience comes with an ownership question: decide which tool will manage Flux resources after bootstrap so you don't have two tools trying to control the same things.

Another option is the Flux Operator. You can install the operator and define a FluxInstance that points at the repository containing your Flux configuration. From there, Flux can reconcile its configuration from Git.

That still leaves an initial installation step to arrange, but it gives you a declarative way to describe the Flux instance without making Terraform its ongoing manager.

It depends on what you mean by not managing it. Terraform can be useful if your team wants the cluster and its initial GitOps installation in the same provisioning workflow. But if you're going to remove it from that role immediately, the extra setup may not buy you much.

Also worth checking what your cluster platform offers before choosing between those two. For example, AKS has a GitOps extension that uses Flux, which is a different management option from bootstrapping it yourself.