I think the streaming for Prime subscribers is to just "get our feet wet" - I should imagine the hope is to reel us in so that we will buy or rent more videos than available for free streaming.
However, just to be sure: When you say there is much selection, I would assume you've (1) scrolled up and down the screen that first appears when you open the app, or (2) scrolled left to right in each category's line or (3) selected "view more" for each category? Yeah, there's not a lot, but there is more than I would have expected.
Also, maybe if you went to the actual site via a web browser you might see more choices. If you find something you like, you can always add it to your queue for viewing later [on the ipad].