This is happening because Outlook versions since 2007 use Microsoft Word to render emails, and it renders restricted support for HTML and CSS.
Below are known issues or bugs with the rendering of newsletter templates
When forwarding an email to outlook, the email does not render the same format in the recipient's email client.
A few email clients have limited support for CSS background images and for some of the properties for controlling how the background image appears, not displaying the background image at all.