This is the reason. The Google Workspace agreements has strong promises about how you data is used. IIRC there is a section about not using your data to improve the service for others. So this means that they would be unable to train the model with data from workspace users (other than fine-tuning which is only used by that specific account). For MVP they don't have the correct labeling and filtering in place to be sure that they aren't using Workspace data in a way that isn't allowed so they take the easy route and disable the service. This is the reason why many Google features and services are not available or are missing features for Workspace accounts.

Of course it would be nice to be able to opt-in either globally or by service and accept that these interactions are not protected with the usual data rules. But this takes even more effort and legal work to implement, so no one has bothered.

A Googler explained it to me once - it's something like as if the Workspace accounts are on a completely different instance of Google, and supporting both in an app is surprisingly hard.

Still, after all this time the geniuses at Google kinda ought to have solved this :)