Why can't I download the model after archiving?
Depends on the specific circumstances / model; as indicated, publishing content related to the model in future is not in violation of the TOS (otherwise the page would be wiped entirely), so it's more likely a matter of distribution of the model by civitai being disallowed for some (legal) reason, or at a someone's discretion.
There's woefully little information on the exact purpose of this archive mode on the repo; it was added in a single commit which is pretty clear about the intended function, but no comments or changelog whatsoever beyond that. https://github.com/civitai/civitai/commit/250322528d0668e81c0791f67bdef258b0346fb7
There might be some introduction to the feature on discord, but I don't know how to search there effectively (too many unrelated "archive" mentions). Justmaier spoke of it early on (27/01), in relation to preserving information (in the form of prompts, reviews, tips) rather than full deletion, for example of old versions of a model, that seems like solid reasoning.
Edit: Okay, found it. Maxfield's notes from 15/04 include the following:
Added two new status for the models:
Archived: (usable by creators) allows the model page to stay up but removes the ability to download the resource. The purpose being if a resource needs to be taken down by a creator for whatever reason, they still have a platform to communicate from
Take-down: (site admin only) [...]
So, it's most likely simply the author's choice, the reason is arbitrary.