We typically recieve more than 50 uploads each day, and each home page display only has 4 slots each.  For 3D models, for example, we check out and select uploads for home page appearance by seeing how popular and how well rated each upload is.  Since we are limited now in the number we can choose, at times some uploads that deserve to be shown on the home page don't make it.  Also, to encourage high quality uploads we try to display the best new uploads in each category.