Udacity has a model in which every class is self paced and they have not deleted a single one since uploading them.

That's because the staff needs a schedule similar to the academic schedule so they can answer questions, correct things, participate in the forum discussions...

Crypto I has been offered several times though (at least 4 or 5). If you ever signed up for one of the offerings, you can still access to the full course (videos, lectures, and I think even the automated grader) as well as the forums (but the forum activity usually fades down after the end of the course).

It's not because of some practical reasons, just university policies.

I'm not sure what you're saying. As far as I know, Crypto II has never been offered, so the problem is probably that they haven't developed any course material for it.