There are a few reasons you could be seeing this issue. Web standards have been updated several times, allowing CSS to make web pages look much nicer. Some email clients have been trying to keep up with those web page formatting standards in their email clients.
Outlook is the email client that has a history of not displaying most email customization. It cannot display buttons with rounded corners, for example, and allows the use of only 8 different fonts.
Other clients, such as Gmail, can handle most CSS customization, but fonts are where most email clients fail, Gmail included.