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The fundamental diff between feature flags and config is the former is meant to be a soft deploy of code where everyone is expected to eventually be on the new code. Thus it should have a timer built in where it stops, and you should consider all new customers launching with it on.

As for why: if you don't deprecate the feature flag in some time span, you're permanently carrying both code paths. With ongoing associated dev and qa resources and costs against your complexity budget.

Permanent costs should only be undertaken after careful consideration, and should be outside the scope of a single dev deciding to undertake them. Whereas flags should be cheap to add to enable dev to get stuff into prod faster while retaining safety.

Permanently making something a config choice should be done after heavier deliberation because of the aforementioned costs, and you often want different tools to manage it. Including something heavier duty than a single checkbox/button in your internal CS admin tooling. These are often tied into contracts or legal needs, and in many cases salesforce should be the source of truth for them. Or whatever CPQ system you're using.