To be honest its simply as issue of space. Mobile/tablet screens are just a lot smaller (duh) that a desktop screen so there's only so much space to go around. If you access the site on a desktop you will see the options to change create an Order Book entry when you buy your chosen stock. You can "force" this to happen on your phone as long as it allows you to display the "desktop site" or enter "desktop mode" (or whatever phrase it uses).
Perhaps in time I'll change it to be available but at the moment it's all about striking a balance between what people need quick access to the most versus things they need every now and then.