Because desktop Outlook renders HTML with Microsoft Word's engine. A quirk that ignores much of modern CSS. Rounded corners, background images, custom fonts, and flexbox layouts all degrade or vanish for Outlook recipients.
Fixes, in order of sanity: send simpler emails (plain-text-style marketing mail sidesteps the entire problem and converts better for service businesses anyway); if you need design, use your platform's templates (they carry Outlook-conditional code) and preview specifically for Outlook before sending; build layouts on tables, never floats.
The strategic read: if your audience is business owners, a meaningful share reads in Outlook. Design for the constraint or, better, write like a person and stop needing the design.