I'll assume you've read this post:
viewtopic.php?f=14&t=92997 (If not, there it is.)
It's mostly newer videos that get the preview option for now. Videos need to be converted to a streaming-friendly format for the preview and I believe that the priority was to have the newest videos and the most popular ones (top 10, VCA winners, etc.) available for streaming first. Some videos may not be available for streaming due to the fact that they haven't gotten around to re-encoding it yet. I'm not exactly sure as to how many videos have been re-encoded but currently doing a super search for videos with previews I get 40913 results of a total of 81561 locally available videos. So approximately half have been encoded for streaming previews.