Polls that are otherwise suitable for the IMDb home page are not featured because the images do not crop well. IMDb can't have images that cut off or have big gaps. People and objects within the image should also be clearly visible and properly centered. Movie posters or portraits tend to work best for IMDb Home Page polls, as IMDb can be sure that the images will fill the space and not get cut off.
The grid view is a good indication of what a poll would look like if it were to be run on the IMDb Home Page. The grid view is most useful for image lists. For the other list types, the default list view generally shows the images in the same format. The first five images are the ones that are used when a poll is run on the IMDb Home Page. When one or more of the first five images do not appear to crop well, consider switching them with images that crop better unless this adversely affects the poll.
For example, this is a recent poll featured on the IMDb Home Page:
This is a link to the grid view for the list for this poll:
To see the grid view for an image list, the author needs to hand edit the link to the list. For example, this is the standard view for the image list for the poll Future 'Star Wars' Titles:
You can get the grid view by replacing #1 with ?view=grid:
You will note, this specific poll would not work well on the IMDb Home Page.