zero replies only goes a short way back in time, why?

The Zero replies list only goes back in time a short way, seemingly as many threads as fit on one page. Why is this?

I'm not sure, but I noticed if I remove one (by answering it) then a new one shows up at the bottom of the page.  Perhaps the original programmers thought one page of zero replies is the right amount?  I know when I go through the list, some of them I don't reply.  With this design those will sink down the list and disappear.

I had assumed this might be an expensive database query.    But I have definitely run out of zero-reply threads I wanted to reply to before I was feeling done in the past; it would work for me if this was a typical paged list that let us go further back in time.

It is that way by design.