A project is designed to be just that, a single item of content or a project completed. The reason for this is that projects are used to give commissioners examples of work you have completed in a style similar to what they are looking for.
A showreel isn't specific enough to make an application with.
For example, if a commissioner is looking for anime-style animation and you recently completed a project in that style, you can share that project with your application, demonstrating to the commissioner that not only have you got an understanding of that style but you have the ability to execute it also. A showreel won't be able to demonstrate this as effectively.