Illustration linked to InDesign document

I created a color illustration in Illustrator and saved it as a .ai file.  I am working on a black and white manual which I created in InDesign.

When I place the illustration into my InDesign document, it looks fuzzy.  When I export my InDesign document as a .pdf file it looks okay. When I print out the page with the illustration it my PDF document looks okay.

Question – why does the linked illustration in my InDesign document look fuzzy?

Thanks,

Matt