Image looks fine in Illustrator but is low quality and pixelated when exported

Hi all, I'm working with this display ad and it looks fine within Illustrator but no matter what I do when exporting it looks come very pixelated (screenshots attached). My document is at 72ppi and the ad size is 300x600. I've tried Save for Web and Export As and neither makes much of a difference. I haven't had this issue with previous ad designs so I'm wondering what's going on