This has nothing to do with bad programming in any manner.
You can put the sidebar on the index under the cat display by turning on the undercats bar in sidebar options.
Lets look at your site so you understand the issue that I see.
Your constricting your page to say 800 pixels wide. Look at the photo on your showphoto page it is 600 pixels wide and that is not even without all the information rating bar comments box etc. There is no way you can show a sidebar on that page based on that page size and photo sizes and expect things to fit.
Now lets go to your index and look at how just looking at the date format your using the avatar column and the category name display your using over 630 pixels wide easy to display that info and then you tried showing the sidebar thats why nothing fits.
There are options in the admin panel to turn off certain columns etc so that you could potentially show a sidebar there. It is just what settings you turn on or off that make you able to do that.